[tphone]

I had this problem when I first installed the excellent GSplayer. It 'took over' the major filetypes in the registry so that it was the default player for most audio files. Most of my my ringtones no longer played, or even vibrated if they were set.
When I unchecked those filetypes in the GSplayer preferences, my problem disappeared.
I suggest that you re-install Mortplayer and check its' default file associations and change them in the programs preferences so that it's the default player for only the filetypes you want it to be.
Now, I am making an assumption that Mortplayer allows you to change them, but I have never used it and don't know if that's an option.
Good luck.