Layers is not a PR feature. It is in PS and AE, but PR uses tracks. Is that what you mean?
If so, then make sure the audio tracks are not turned off / muted.
MP3 Audio can cause all sorts of problems. After you check out Harm's suggestion to check the "Visibility" of that/those Audio Track(s), in the Track Header and in Audio Mixer, if all is fine there, and that/those Track(s) are Enabled, try doing a Render/Replace for the MP3's, to convert them to WAV's.
I always convert all MP3's to PCM/WAV 48KHz 16-bit, prior to Import, and never have issues. Others find that some MP3's are fine, but others just do not function, though there were not errors on Import. MP3's can be odd, troublesome files.
Thanks, the file in question …as it turns out was only one of the tracks (not “layer”…sorry), albeit the main track.
I found it as a low res sample off the net. I then slowed it down to 17% - and it sounded great on the timeline. I’ve used it in 2 projects. The first one had no problem.
However in the project in question, no matter what I tired –converting it to WAV in Premiere or whatever, It would play fine on the timeline, but not play when exported (to any format). I took the same file and re rendered it in Soundbooth as a stereo file –both mp3 and the much larger wav, and tied it again. Both files then would export…but the sound mysteriously changed pitch (without using any effects). I then threw up my hands and used Windows sound recorder. This worked. I converted the Windows media file to a wav and mp3 files…and they both work. Very odd!
BTW I prefer mp3 since it saves space, and I’ve had no inherent problem with them and cannot tell the difference with the audio with high quality Mp3s.