content type for mp3 download response

Try "Content-Type: audio/mpeg"

Update

To encourage the browser to download the mp3 rather then streaming, do

Content-Disposition: filename="music.mp3"'

.mp3    audio/mpeg3
.mp3    audio/x-mpeg-3
.mp3    video/mpeg
.mp3    video/x-mpeg

Using Content-Disposition: attachment... forces a download box to appear instead of having to right click -> save target as.