Had the same problem and seems to be fixed by:
In the slideshow module go to /PLAY menu/content , and check "use selected photos" option. If the "use all" option is checked it will not recognise music files. Odd.
Thanks for taking the time to respond. I don't think this was my problem. Besides the m4a audio file/slideshow problem, Lightroom wouldn't recognize any .mp4 files (which it had been doing). After consulting with Adobe support, I opened a New Catalog, and chose "Import from another catalog" (that other catalog being my old one). Both audio and video issues were fixed. Again, thanks for your help.