Do the avi's play the audio in the Media Library but not in the timeline tracks? If so, try and extract the audio with a right click and then place that audio in the track - it'll be a .wav file.
My guess is the avi's (it's just a container) has an audio that is compressed to a format PDR doesn't like. If the avi is from a video screen capture - choose a different setting when you capture.
Please provide the MediaInfo see Part J in the guide at the top of the forum/sticky.
Dafydd