Nothing to go on.
What is your source, codec, seqence settings, pc specs?
Read Bill Hunt on a file type as WRAPPERhttp://forums.adobe.com/thread/440037?tstart=0
What is a CODEC... a Primer http://forums.adobe.com/thread/546811?tstart=0
What CODEC is INSIDE that file? http://forums.adobe.com/thread/440037?tstart=0
Report back with the codec details of your file, use the programs below
For PC http://www.headbands.com/gspot/ or http://mediainfo.sourceforge.net/en
For Mac http://mediainfo.massanti.com/
Also, have you used Copy/Paste with ANY of these problem Source Files?
If so, then that operation can lead to lost (actually flatlined) Audio in muxed files.