Sound problems

Whilst watching our opening sequence through we have been noticing errors wiitht he sound, at all times there is a slight buzz in the background of the camera; at the start where there shots of the area are being shown we have been able to turn the sound down as we have the music that we spent a while choosing as we wanted one that suited the storyline of the film and we decided that the one that is on suited the film best, we are pleased with this choice. However when the camera switches to the conversation the buzz of the camera is still there, unfortunately we cannot turn down the sound as if we did this the conversation would be inaudible. We have considered recording the audio again and putting it on top of the film but we thought that if we did this then the mouth movements would be different and would cause our opening sequence to lack verisimilitude.