are you sure your avahi-daemon is running?
The port is already in use, so maybe you are restarting an instance of mt-daapd, or have other DAAP servers running (banshee, rhythmbox?)
Other than that, mt-daapd runs as nobody/nobody. So make sure that user has read access to the mp3 files, and read & execute to the directory tree beneath your music_dir 😉