Running Other applications

.mp3 file is the parameter of the wmp.exe,  you really should indicate the full path for the .mp3 file.but one thing, there should be some "\" characters in the path, which will be treated as ESC, so you should add "@" beofore the path. For example:

argumets = @"C:\MUSIC\123.MP3"

hope this can help.
Good luck

At first you should reference the namespace  System.Diagnostics

then  using an instance of one method,just like below:
Process WMP= new Process();
    WMP.StartInfo.FileName   = "WMP.exe";
    WMP.StartInfo.Arguments = "123.mp3"
    WMP.Start();

You can try, this works for me.