Welcome to the forum.
I agree with Jim, that a lot more info will be needed. While you are gathering up that info, here are some specific questions, that might be useful too:
Do you have a Waveform Display for the Clips, where you hear no Audio, or is that straight line, i.e. empty?
Did you allow your Audio to be completely Conformed? See this article: http://forums.adobe.com/message/3305940#3305940
If this is Audio from an MTS AVCHD file, did you Copy the full folder structure from your SD card, or just the media files? If only the media files, you will want to Copy the full folder structure, and Import those media files again.
Good luck, and please let us know a lot more,