After it a voice roareth - After the lightning; that is, the flash is seen before the thunder is heard. This is apparent to all, the interval between the lightning and the hearing of the thunder depending on the distance. Lucretius, who has referred to the same fact, compares this with what occurs when a woodman is seen at a distance to wield an axe. The glance of the axe is seen long before the sound of the blow is heard:
Sed tonitrum fit uti post antibus accipiamus,
Fulgere quam cernunt ocuil, quia semper ad aures
Tardius adveniunt, quam visum, guam moveant res.
Nunc etiam licet id cognoscere, caedere si quem
Ancipiti videas ferro procul arboris actum.
Ante fit, ut cernas ictum, quam plaga per aures
Det sonitum: Sic fulgorem quoque cernimus ante.
Lib. vi.
He thundereth with the voice of his excellency - That is, with a voice of majesty and grandeur.
And he will not stay them - That is, he will not hold back the rain, hail, and other things which accompany the storm, when he begins to thunder. "Rosenmuller." Or, according to others, he will not hold back and restrain the lightnings when the thunder commences. But the connection seems rather to demand that we should understand it of the usual accompaniments of a storm - the wind, hail, rain, etc. Herder renders it, "We cannot explore his thunderings." Prof. Lee, "And none can trace them, though their voice be heard." According to him, the meaning is, that "great and terrific as this exhibition of God's power is, still the progress of these, his ministers, cannot be followed by the mortal eye." But the usual interpretation given to the Hebrew word is that of "holding back," or "retarding," and this idea accords well with the connection.

After it a voice roareth - After the flash has been seen, the peal is heard; and this will be more or fewer seconds after the peal, in proportion to the distance of the thunder cloud from the ear. Lightning traverses any space without any perceivable succession of time; nothing seems to be any obstacle to its progress. A multitude of persons taking hands, the first and the last connected with the electric machine, all feel the shock in the same instant; and were there a chain as conductor to go round the globe, the last would feel the shock in the same moment as the first. But as sound depends on the undulations of the air for its propagation, and is known to travel at the rate of only 1142 feet in a second; consequently, if the flash were only 1142 feet from the spectator, it would be seen in one second, or one swing of the pendulum, before the sound could reach the ear, though the clap and the flash take place in the same instant, and if twice this distance, two seconds, and so on. It is of some consequence to know that lightning, at a considerable distance, suppose six or eight seconds of time, is never known to burn, kill or do injury. When the flash and the clap immediately succeed each other, then there is strong ground for apprehension, as the thunder cloud is near. If the thunder cloud be a mile and a half distant, it is, I believe, never known to kill man or beast, or to do any damage to buildings, either by throwing them down or burning them. Now its distance may be easily known by means of a pendulum clock, or watch that has seconds. When the flash is seen, count the seconds till the clap is heard. Then compute: If only one second is counted, then the thunder cloud is within 1142 feet, or about 380 yards; if two seconds, then its distance is 2284 feet, or 761 yards; if three seconds, then 3426 feet, or 1142 yards; if four seconds, then the cloud is distant 4568 feet, or 1522 yards; if five seconds, then the distance is 5710 feet, or 1903 yards; if six seconds, then the distance is 6852 feet, or 2284 yards, one mile and nearly one-third; if seven seconds, then the distance of the cloud is 7994 feet, or 2665 yards, or one mile and a half, and 25 yards. Beyond this distance lightning has not been known to do any damage, the fluid being too much diffused, and partially absorbed, in its passage over electric bodies, i.e., those which are not fully impregnated by the electric matter, and which receive their full charge when they come within the electric attraction of the lightning. For more on the rain produced by thunder storms, see on Job 38:25 (note). This scale may be carried on at pleasure, by adding to the last sum for every second 1142 feet, and reducing to yards and miles as above, allowing 1760 yards to one mile.
He thundereth with the voice of his excellency - גאונו geono, of his majesty: nor is there a sound in nature more descriptive of, or more becoming, the majesty of God, than that of Thunder. We hear the breeze in its rustling, the rain in its pattering, the hail in its rattling, the wind in its hollow howlings, the cataract in its dash, the bull in his bellowing, the lion in his roar; but we hear God, the Almighty, the Omnipresent, in the continuous peal of Thunder! This sound, and this sound only, becomes the majesty of Jehovah.
And he will not stay them - ולא יעקבם velo yeahkebem, and he hath not limited or circumscribed them. His lightnings light the world; literally, the whole world. The electric fluid is diffused through all nature, and everywhere art can exhibit it to view. To his thunder and lightning, therefore, he has assigned no limits. And when his voice soundeth, when the lightning goes forth, who shall assign its limits, and who can stop its progress? It is, like God, Irresistible.

After it a voice roareth: he thundereth with the voice of his excellency; and he will not stay (c) them when his voice is heard.
(c) Meaning, the rains and thunders.

After it a voice roareth,.... After the lightning comes a violent crack or clap of thunder, which is like the roaring of a lion. Such is the order of thunder and lightning, according to our sense and apprehension of them; otherwise in nature they are together: but the reasons given why the lightning is seen before, and so the same in the flash and report of a gun, are, because the sense of seeing is quicker than the sense of hearing (y); and the motion of light is quicker than that of sound; which latter is the truest reason (z). The roaring voice of thunder may be an emblem of the thunder of the law; its dreadful volleys of curses, vengeance, and wrath on the breakers of it, as delivered out by Boanergeses, sons of thunder, Mark 3:17, or the loud proclamation of the Gospel, made by the ministers of it; and the alarming awakening sound of the word, when attended with the Spirit and power of God, to sinners asleep and dead in trespasses and sins; upon which they awake, hear, and live;
he thundereth with the voice of his excellency: that is, God thunders with such a voice, an excellent and majestic one; for his voice of thunder is full of majesty, Psalm 29:4. So is the voice of Christ in the Gospel; he spake when on earth as one having authority, and he comes forth and appears in it now with majesty and glory; and speaks in it of the excellent things which he has done, of the excellent righteousness he has wrought out, of the excellent sacrifice he has offered up, and of the excellent salvation he is the author of;
and he will not stay them when his voice is heard; either the thunder and the lightning, as some; which he does not long defer after he has given out the decree concerning them, the order and disposition for them: or rather the rain and hail; these are not stayed, but quickly follow the flash of lightning and clap of thunder: "for when he utters his voice of thunder, there is a multitude of waters in the heavens"; and these quickly come down and are not stopped, Jeremiah 10:13. The word for "stay" signifies "to supplant", or "act deceitfully"; the name of Jacob is derived from this root, because he supplanted his brother, Genesis 25:26; and so it may be rendered here, "he will not supplant", or "deceive them (a), when his voice is heard": that is, either he does not subvert them, the heavens and earth, but preserves them; though he makes them to tremble with his voice of thunder (b): or he does not act the part of a secret, subtle, and deceitful enemy, when he thunders; but shows himself openly as a King, executing his decrees with authority (c): or rather he deceives none with his voice; none can mistake it; all know it to be the voice of thunder when it is heard: so Christ's sheep know his voice in the Gospel, and cannot be deceived; the voice of a stranger they will not follow, John 10:4.
(y) Senec. Nat. Quaest. l. 2. c. 12. so Aristot. Meteorolog. l. 2. c. 9. (z) The noise is commonly about seven or eight seconds after the flash, that is, about half a quarter of a minute; but sometimes much sooner, in a second or two, or less than so, and almost immediately upon the flash: this is when the explosion is very near us. Philosoph. Transact. abridged, vol. 2. p. 183. see vol. 4. p. 398. (a) "non supplantabit ea", Munster; so Schmidt, Michaelis, Gussetius, p. 633. (b) So Schmidt. (c) So Gussetius.

The thunderclap follows at an interval after the flash.
stay them--He will not hold back the lightnings (Job 37:3), when the thunder is heard [MAURER]. Rather, take "them" as the usual concomitants of thunder, namely, rain and hail [UMBREIT] (Job 40:9).

After - After the lightning, which is seen before the thunder is hard. Them - The lightnings spoken of in the beginning of the verse.