Item Id: 1466
For Update 18.104.22.168 released March 31/2017
For the moment only Unturned's theme song is available officially, but adding custom music is relatively straightforward. Export audioclips from Unity into an assetbundle ending with .content. Then create a stereo song asset from the asset browser (right click) and drag/drop the audioclip into the song slot. The theme song example file can be found in Bundles/Assets/Songs.
Added in update: 22.214.171.124