raid1000, people doesn't use sound to play music because of several reasons. I going to mention 3 of it.
1) If there is any other sound been play, there is a small possibility of the sound overwrite the music since it was play as a sound rather than music. This situation sometimes depends on the computer.
2) If you minimize your wc3, it would immediately stop the music since it was counted as sound and not music. If it was music, it would continue.
3) Setting it as sound would make the music louder, but it would also drown out the conversation sounds and other sound such as sound effects. That is why there is a purpose of the music to have lower sound compare to others.