"And the men which 
      journeyed with him stood speechless, hearing a voice, but seeing no 
      man." -Acts 9:7
      "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." -Acts 22:9
      
      
          In the Encyclopedia of Bible 
          Difficulties by Gleason L. Archer, the following explanation is 
          given: 
          
            ". . . In the original Greek, 
            however, there is no real contradiction between these two 
            statements. Greek makes a distinction between hearing a sound as a 
            noise (in which case the verb "to hear" takes the genitive case) and 
            hearing a voice as a thought-conveying message (in which case it 
            takes the accusative). Therefore, as we put the two statements 
            together, we find that Paul's companions heard the Voice as a sound 
            (somewhat like the crowd who heard the sound of the Father talking 
            to the Son in John 12:28, but perceived it only as thunder); but 
            they did not (like Paul) hear the message that it articulated. Paul 
            alone heard it intelligibly (Acts 9:4 says Paul ekousen phonen--accusative 
            case); though he, of course, perceived it also as a startling sound 
            at first (Acts 22:7: "I fell to the ground and heard a voice [ekousa 
            phones] saying to me," NASB). But in neither account is it 
            stated that his companions ever heard that Voice in the accusative 
            case." 
            -- Encyclopedia of Bible Difficulties, by Gleason L. Archer, 
            p. 382. 
          
          According to Archer, this 
          distinction does indeed exist in the Greek language.