Acts 9:7 says that the men around Paul “stood speechless because they heard the voice but saw no one.” (NRSV)
In Acts 22:9 Paul says they “saw the light but did not hear the voice of the one who was speaking to me.”
Is this a formal contradiction, or merely a superficial contradiction that can be harmonized? Robert Bowman summarizes:
“Evidently Luke [the author of Acts] means to convey that Paul’s companions saw a light and heard the sound of someone’s voice coming from the light, but only Paul saw the person in the light and heard the words spoken by the voice of that person. This explanation is reasonable, plausible, consistent with the wording of the texts, and supported by contextual elements in both accounts and in the third account found in Acts 26.”
He gives four supporting reasons:
1. We should give Luke, an “author who demonstrates high literate skill and care,” the benefit of doubt, and prefer an available plausible explanation.
“There is a reasonable, general presumption that a literate, careful author more than likely is not contradicting himself in the same book when recounting something more than once… If a plausible explanation is available that would clear up the discrepancy, it should be preferred in such cases to the claim that the author has clumsily contradicted himself.”
2. The hear vs. comprehend distinction parallels the issue of what they saw.
“This distinction would neatly parallel the distinction the two texts make with regard to what Paul’s companions saw: they saw the light (22:9) but did not see the person whom Paul saw in the light (9:7). Thus both the sound and the light were indistinct or unidentifiable for Paul’s companions but were perceived and understood by Paul as the voice and appearance of a figure who identified himself as Jesus.”
3. “Hear” has a range of meaning.
“The inherent ambiguity and range of connotations of the words for hear in Greek, English, or any other language makes this interpretation eminently plausible. We have all had occasions, for example, when we could “hear” someone speaking but we could not “hear” their words well enough to understand what was said. (We tend to have this experience more frequently as we get older!) Thus, the issue is not even really how the word ἀκούω should be translated but how it should be understood in context.”
4. That Jesus was speaking directly to Paul was a point of emphasis.
“All three parallel accounts in Acts of Paul’s vision make the point that the voice of Jesus was speaking directly to Paul and that Paul, specifically, heard what Jesus said. Luke reports that Saul “heard a voice speaking to him” (αὐτῷ, 9:4). In both the second and third accounts, Paul states, “I heard a voice speaking to me” (πρός με, 22:7; 26:14). In the second account, Paul also refers to the voice “of the one who spoke to me” (τοῦ λαλοῦντός μοι, 22:9). This redundancy in the second account is almost certainly for emphasis: Luke, in quoting Paul’s testimony of what happened, reports him referring twice to the voice as one that was speaking specifically to him… The voice was spoken to Paul, for his benefit—for his ears—consistent with its not having been meant to be understood by Paul’s companions.”
These four reasons are then perhaps supplemented by the grammatical possibility that the cases used in Greek for “voice” (genitive in 9:7, accusative in 22:9) encourage a distinction. Bowman advises however a “carefully qualified application of the grammatical distinction”, one that does not depend on an overstated grammatical rule.
“The popular grammatical argument based on classical usage of the genitive and the accusative is too weak to serve as strong evidence for this interpretation, let alone a proof. However, as [scholars] Young, Witherington, and Keener all suggested, that classical distinction might just happen to apply in this instance. The contextual factors reviewed above lend good support to that suggestion.”
Bowman cites A. T. Robertson:
“In 22:9 Paul says that the men “beheld the light” (to men phōs etheasanto), but evidently did not discern the person. Paul also says there, “but they heard not the voice of him that spake to me” (tēn de phōnēn ouk ēkousan tou lalountos moi). Instead of this being a flat contradiction of what Luke says in 9:7 it is natural to take it as being likewise (as with the “light” and “no one”) a distinction between the “sound” (original sense of phōnē as in John 3:8) and the separate words spoken. It so happens that akouō is used either with the accusative (the extent of the hearing) or the genitive (the specifying). It is possible that such a distinction here coincides with the two sense of phōnē. They heard the sound (9:7), but did not understand the words (22:9).”
— Archibald Thomas Robertson, Word Pictures in the New Testament, Vol. III: The Acts of the Apostles (Nashville: Broadman Press, 1930), 117-18.
More from Robertson on Acts 22:9:
“The accusative here may be used rather than the genitive as in verse 7 to indicate that those with Paul did not understand what they heard (9:7) just as they beheld the light (22:9), but did not see Jesus (9:7). The difference in cases allows this distinction, though it is not always observed as just noticed about 22:14 and 26:14. The verb akouō is used in the sense of understand (Mark 4:33; I Cor. 14:2). It is one of the evidences of the genuineness of this report of Paul’s speech that Luke did not try to smooth out apparent discrepancies in details between the words of Paul and his own record already in ch. 9.”
— Ibid., 390
Bowman notes that Robertson’s appeal to the distinction is probabilistic given the context:
“It is even clearer in Robertson’s Word Pictures that he is not appealing to the distinction between the uses of the genitive and the accusative as a rule. Instead, he argues that the distinction is likely with regard to Acts 9:7 and 22:9 based on the contexts.”
 “Heard but Not Understood? Acts 9:7 and 22:9 and Differing Views of Biblical Inerrancy.” Paper presented at the Evangelical Theological Society annual meeting, Providence, RI, November 15, 2017.
I am reminded of John 12:29:
“The crowd that stood there and heard it said that it had thundered. Others said, ‘An angel has spoken to him.'”