Probably because the default program to open them is the wrong one. You need to change the default back to your normal audio player. Hopefully that's not WMP which doesn't always have the correct codecs to play all media files.
(a) Setting default audio program. But also check you have correct audio codecs for your player. You still haven't told us what your normal player is.
(b) CODECs list and alternate players for Audio & Movies