The files are all:

.mp3 .mp4 .wma .wmv

They are all in the same folder as the .pptx which has other similar files encrusted that open with no problem.

The files that do not open, play perfectly outside the Powerpoint presentation, but are dead when encrusted. I tried eliminating them, and reinserting, but there was no change.

I was told to make sure the file names contain no unusual symbols, but there were none. Might a newer version of Powerpoint solve the problem?

Surely I can't be the only one with this problem. Thank you for any and all help.