I use WMP too and the same happened to me.
The files that played in Realplayer were m4A so what I did was this;
Open windows explorer[via start/programs/accessories] click on tools at the top, then folder options. In the
file types tab, scroll down to m4A and highlight it and hit the
change button and choose the program you want it to open with[in this case, WMP] Then check the box for
always use this etc etc. The same can be done with some other different file types and different programs.
