This is a great question. There are three different account of Paul's conversion in the book of Acts.
The first one comes from Acts 9. We can say that this is Luke's account of Paul's conversion. Hence, it comes from Luke's point of view. The basic flow is: Paul's mission, persecution of the fledgling church, conversion, post conversion proclamation, and the persecution of the Jews. The most important point to remember in this account is that it is said that the people heard the voice of God (Saul, Saul, why are you persecuting me?) in Paul's encounter with Christ on the road to Damascus.
The second account comes to us in Acts 22. The basic contours are the same, but this time, it is said that the people did not hear the voice of God. Scholars have spilled a lot of ink trying to harmonize this!
The third account comes to us in Acts 26. Again it follows the basic pattern, but this time it does not say anything about whether the people heard the voice of God or not.
The key point to remember is that these accounts are from different perspective, as they are seeking to underline different points. When we realize this, details are not as important. What is important are the theological points that are being made.