Yes, ppl have reported an assortment of probs with media player/codecs. I have had a prob occasionally myself since i accidently let the lastest version of MP install..but thats when i wont let it access the web. I forget the exact details, but unless MP can access the web, some things in it wont work right. (gee, would MS really do something like that? *snort* )
Keep in mind that mIRC itself does NOT play the mp3s, it only sends the request to windows, which then pokes MP.