SW: When loading a level from the main menu, try play a wave/redbook music track

On a typical scenario of the user having the retail version of the game and redbook/wave music available, the music won't actually start playing if the user loads a level from the main menu, without having started a game already.

Passing -1 for the cdaudio_track parameter of the function PlaySong() when loading outside the game loop was the cause of this. Added a function called PlayLevelSong() which correctly picks the right track.

I tested with the Shareware release and MIDI playback still works as expected.