These texts both report on the experiences of the people who were with Paul during his conversion on the road to Damascus. Acts 9:7 says that these people heard a voice but saw no one; Acts 22:9 says that they saw the light, but did not understand the voice. The one says they heard but did not see anyone, and the other says that they saw a light but did not hear.

Generally, we give any author or speaker — biblical or otherwise — the benefit of the doubt with regard to internal contradictions. When there is a way to understand his meaning as compatible with itself, we should be charitable and assume that interpretation unless we have reason to think otherwise. People commonly say things that can be construed as contradictions, but they frequently don't intend those interpretations of their words. Since both the passages in question come from the same work, it should incline us to think that Luke saw them as compatible and not contradictory.

In any event, there is no contradiction between seeing a light and not seeing a person. The problem is that Acts 9:7 says that they "heard" (from the Greek akouo), whereas Acts 22:9 says that they did not "hear" (also from the Greek akouo). Many translations (e.g., NIV, NASB) reconcile these passages by saying that in the first passage akouo refers to hearing a sound or voice, while in the second passage it refers to understanding that same sound or voice. In other words, they heard it, but did not understand what it was saying.

This is certainly within the realm of possibility. Akouo may mean many things, including "hear," "understand," "obey," "listen," "heed," etc. And there is no reason to insist that because Luke (the author of Acts) meant one thing in one passage, he must have meant the same thing by the same word when he used it in a similar context latter in the same work. Often, authors maintain consistency with regard to meaning in similar contexts, but not always. This may be one of those "not always" times rather than one of the usual times.

It is also possible that Acts 9:7 should be translated "hearing his voice" rather than "hearing the voice." It was very common in Greek to use the article (i.e., "the" in English) as a possessive pronoun (e.g., "his"). In this case, the reference might well be to Paul's voice rather than to Jesus' voice. That is, the people saw the light and heard Paul speaking to someone, but they didn't hear anyone speaking to Paul. Other solutions are also possible, so it is not necessary to see these passages as contradictory.