Printed from http://tektonics.org/paulthree.php
In the book of Acts we have not one, not two, but three accounts of Saul's conversion. Here's a sample from the major focus points:
Acts 9:7 And the men which journeyed with him stood speechless, hearing a voice, but seeing no man.
Acts 22:9 And they that were with me saw indeed the light, and were afraid; but they heard not the voice of him that spake to me.
A standard solution in this regard says either that:
However, there are obvious and simple solutions that have nothing to do with language.
The first way to look at the solution is that Paul, when retelling his story in Chapter 22 (and 26), is incorrect when he says that his companions didn't hear the voice (and perhaps on other matters of difference), and Luke is merely quoting him.
Does this mean that Paul was lying or unreliable? No, it means that he was misinformed at the time of his speeches in Chapters 22/26. We cannot simply assume that Paul had the correct information, and the possibility exists, for example, that his companions did not tell him that they had heard the voice. Indeed, they may have DENIED hearing it, both to Paul and in his presence, for fear of the implications of what they had heard. We might expect this is they were the sort of "travelling goon squad" implied - I know from working experience in the prisons that this type of person will tell any lie, no matter how outrageous and obvious, in order to evade an issue.
It is interesting that:
However, another way to look at this is as an intentional contradiction designed to magnify Paul and put down the importance of his companions. This aspect would find parallels in Greco-Roman rhetorical methods; see on this matter Ronald Witherup, "Functional Redundancy in the Acts of the Apostles" from Journal for the Study of the New Testament 48, 1992, pp. 67-86.
This is in line with the suggestion from E. P. Sanders, in his book The Historical Figure of Jesus, that Luke may have intentionally made the stories different, and Wright's comment in The Resurrection of the Son of God that the differences are "best explained by Luke's following a hellenistic convention of style according to which variation in a narrative lends interest." 
Consider what Sanders says:
The author of Luke/Acts was not stupid; he doubtless knew that his stories varied. He could have told the same story the same way, but that would not have been as good a narrative. Like many other authors, both ancient and modern, he disliked repetition; like other ancient authors, he would change events in order to avoid it.
We may not choose to agree with Sanders totally, but we may agree on this much: Luke was not stupid. It strains credulity to suggest that he made an "error" such as is suggested and didn't even realize that it was there.
Tekton reader Dennis Hou has made a very pertinent observation giving a good reason for at least one important functional difference -- an allusion by Luke to an OT passage. Here is what he says:
Deuteronomy iv, 12: "And the LORD spake unto you out of the midst of the fire: ye heard the voice of the words, but saw no similitude; only ye heard a voice."
Acts ix, 7: "And the men which journeyed with him stood speechless, hearing a voice, but seeing no man."
It may worthwhile to consider a possible allusion (or even a typological connection) here.
For starters: Stephen stated, "Behold, I see the heavens opened, and the Son of man standing on the right hand of God," (Acts vii, 56.) Among other things, the standing up includes Psalms lxxxii, 6: "Arise, O God, judge the earth: for thou shalt inherit all nations, [i.e. Gentiles.]" We see the beginnings of this "inheritance" (elsewhere, His people, Israel, are described as the inheritance) with the Ethiopian eunuch.
The Deuteronomy passage refers to the revelation of the Mosaic law. Seeing that the new exodus (A.D. 30-70) of the new Israel (the church) out of the new Egypt (national Israel, e.g. Revelation xi, 8) is about to commence, Paul here takes the role of the new Moses (cf. Romans ix, 3, which alludes to Exodus xxxii, 32) preparing to declare the new law and the new covenant.
In that case, there seems to be good reason for Acts ix, 7 to hint at Deuteronomy iv, 12.