A dumb ass speaking with a man's voice only makes sense because the ass is not a man. Think of what it implies about Jesus speaking with an archangel's voice -- although I don't think this is what 1 Thessalonians means, because only a circumstantial or temporal (cf. 1 Corinthians 15) interpretation provides a coherent function for the three consecutive en (combining different functions of the same preposition in the same sentence is usually avoided, e.g. 'he said in anger and in Italian').
Although different in detail Matthew 24:30f offers one the best canonical parallels for the basic Pauline scenario (which differs also in its related forms in 1 Thessalonians and 1 Corinthians): "Then the sign of the Son of Man will appear in heaven, and then all the tribes of the earth will mourn, and they will see 'the Son of Man coming on the clouds of heaven' with (meta)power and great glory. And he will send out his angelswith (meta)a loud trumpet call (salpiggos megalè), and they will gather (episunagô, cf. episunagôgè in 2 Th 2:1) his elect from the four winds, from one end of heaven to the other."