Several answers applicable to attaching a tune to an email, but the original question included "how do I make it play as background music to the email?"

I have occasionally received email with automatically playing background music; but since I consider it rude and delete them immediately (nearly always without reading) I don't have a sample to examine how it was done.

If the original poster intended to ask how to include autoplay background music, the answers given appear to be incomplete.(?)

John