To control default file formats to be used with Windows Media Player
click the start orb and type
default programs
and hit enter
from the resultant window select
Set your default programs
In the left column select
Windows Media Player
and then click the Choose defaults for this program link
If you have previously chosen to use Windows media player as your systems default player (Set this program as default) then your options her will likely be all selected and grayed out so you'll not likely be able to change them.