Problem when playing music in windows media player.?

Hi i have just upgraded to vista home premium from xp media centre and i have noticed now that when i play my music using media player 11, the song fades in and out like someone is turning the volume up and down. I have updated my sound card drivers and i cant reinstall or uninstall media player!! Any suggestions? This is with various audio files, mp3 wma etc. I have used nero to play a song and that played fine its only media player.