Here are two stories related to us by the author of gLuke. Actually, it is the same story, twice, with somewhat variant details. In the first account, it is said that Paul traveling companions "stood speechless, hearing a voice, but seeing no man" while in the second one it is said that they "saw indeed the light, and were afraid; but they heard not the voice of him that spake" unto Saul of Tarsus.
The only possible apologetical approach to this problem is to claim that the verb signifying hearing means something different in each story, perhaps merely "hearing" in the first case but "comprehending" in the second. One could well imagine Paul's companions hearing an indictinct sound like the adults speaking in Charlie Brown animated specials. It would seem that this approach to reconciling these verses is the one favored by the NASB.
The problem with this solution is that the same verb is used in both places and there is no reason from context to believe that hearing with comprehension is not implied in both places. Indeed, I've yet to see any exmaples in the NT in which ἀκούω is taken to mean hearing a voice without making out the words. Even in verses like Mark 4:12 is it taken as read that the listeners are understanding the words and the surface meanings of the parables, but not the profound meanings thereof. Unless such examples prove forthcoming, it is the utmost in special pleading to claim that this word means "hearing a voice without catching the words" in just this one place, given how often it is used throughout the NT to denote hearing voices.