I am trying to create an app that will perform actions on specific times (much like the Windows Task Scheduler). I am currently using Process.Start() to launch the file (or exe)
proc = Process.Start(filename)
should work, but like you say, it returns null instead of a process.
That seems to be inherent to Windows Media Player. Other applications return the process. You can get Windows Media Player's process by specifying the application in the start method.
proc = Process.Start("C:\Program Files\Windows Media Player\wmplayer.exe", filename)
Then you can kill it normally.
proc.Kill()
You will probably need to get the location of the application assiciated with .mp3 files from the registry.