For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Parkinson AZAGBA
Acts 9:7 (NIV) says- The men travelling with Saul stood speechless; they heard the sound but did not see anyone.
Acts 22:9 says- My companions saw the light, but they did not understand the voice of him who was speaking to me.
If they did not understand the voice of him who was speaking to Saul, it means that they heard the voice. They could not interpret the voice of the person speaking. This may be so because the message was intended for Saul alone. That they saw the light and Saul falling down (Acts 22:7) was enough for Saul's followers to know that something serious had happened to their master.