I have difficulties playing some video files! please help...

O.K. so here’s the problem:
when i play a video file with an ac3 audio codec, I cannot here any sound and can only see the video! Gspot says I have a right decoder installed - “Nero Digital Audio Decoder”... I guess there’s something wrong with it!
Do you know what the problem is?
please, please, please help!!! it’s really improtant!