Local music playback on an Xpenology with an USB sound card

Xpenology is an incredible project allowing to turn an old computer into a “Synology” NAS in order to test it. I say “test” because the legality around the project is unclear: while synology publishes its source code (http://sourceforge.net/projects/dsgpl/), not everything is considered as “open”, and officials at Synology considers that …