sounds like all the differant players are trying to compete for the right to play the files and thus causing problems, I have found in the past that if quicktime and media player are installed then media player crashes!, in media player go into files and set as default player for all files (under file types). this should (touch wood) sort it out. If not, best bet is to remove real player, winamp etc (including registery entrys- use reg clean or similar) download jet audio and set that as default player, it will play all files including audio and dvd if you tell it to.
Also make sure that you have got the latest version ofr media player including all the codacs.
Hope this helps