Does this now happen with ALL audio files? Did you change your OS?
Thanks for your reply. No changes to my system. The audio file plays fine in other apps. No issues with the audio in other apps such as Premiere.
The only (very remote) possibility is that this could be one of the phantom issues related to Master Collection installs. Ever since MC CS4 I found that days or weeks after installing the suite, odd and completely random weirdnesses would gradually appear in the system. It could be literally anything: a font not displaying correctly, Illustrator smearing an image, a disappearing cursor in Photoshop, unexplained crashes with Audition, weird pattern on a monitor during startup, an Adobe app taking a long time to quit after all files are closed, etc.
There is a near total lack of coordination between Adobe product teams. The result is that several of the installers iin the MC suite would install tiny little resource files with identical names in identical locations, sometimes with an older dated version installing over a newer version from one of the other app installers. Reinstalling everything from scratch onto a zeroed startup disk would fix things for a while but after a couple of weeks completely new and seemingly unrelated weirdnesses would appear (I am fastidious about my system, my browsing and my system scans - I spent a decade with top agencies in the Chicago loop and I am extremely careful.). In any case, I went through this slash and burn thing several times over the past six years. My last install has survived for over a year with virtually no weirdnesses.
Again, this is a remote possibility, but I've has no such audio issues with AE with other projects over the past year so for it to suddenly do this makes me want to blame evil spirits from San Jose.