My earlier reported problems with BubbleUpnp and flac-files and your recent posting got me busy. Seems that BubbleUpnp is able to play local flac (16 and 24 bits) properly on Volumio. This leads me to the synology settings in mediaserver (DSM 5.0). There is a setting in mediaserver which forces transcoding (“enable audio transcoding”). I had this enabled in order to play files on my old mediaplayer. I disabled it and no longer the upmpdcli/MPD software on Volumio seems to have a problem playing flac files!
Looks like upmpcli/MPD isn’t able to play transcoded files properly but only accepts the original format. This might be the problem with your UPNP server.
The other issue left is that synology’s audiostation isn’t able to see Volumio as an upnp device (to know if you facing airplay or UPNP you have to change the name, see my other post). I restarted my NAS and there it was: audiostation shows both VolumioUpnp and VolumioAir!
Next problem: MPD library showed up empty. Restarting Volumio makes VolumioUpnp disappear in audiostation. Only a stop and start of mediaserver was enough to get it back. But this empties the library again… An update MPD database fixed this. So next time: restart Volumio > restart mediaserver > update MPD
Also fixed foobar/foo_upnp by making a profile for Volumio in “streaming profile” settings and setting decoding to PCM to Never (use profile when User-Agent contains upmpd seemed to work).