Android 5.1 head unit >>> USB Audio (DAC) output

Hi, I installed a MTCD Android head unit with Android 5.1.1 in my car two weeks ago. (I could'nt start this thread in the MTCD section because I'm new here)

My car audio setup goes as follow: Android Head Unit >>> USB Cable >>> PCM2704 USB to SPDIF adapter >>> SPDIF coax cable >>> MiniDSP 6x8 >>> 3 Amps >>> Speakers + sub.

So far I've been able to get SPDIF sound output with Neutron (Direct USB mode), Onkyo HF Player, USB Audio Player Pro, HibyMusic Player.
Sound is amazing btw.
If I understand correctly, all these apps are working with the USB DAC because they use their own app's USB Driver.

Important to know: All song in 44khz are playing fine, songs in 48khz makes every app freeze. I got a way around this by setting Neutron to resample every song in 44khz, so all songs are working now.

Now, if I play a song in Google Play Music, Spotify, Kodi, etc no sound comes out ... same thing for my HU video app, phone calls etc.

Is there a way around this ? What modifications can be made so every sound outputs to the USB Audio ? IE. kernel update, system files modifications etc ??

Thanks for any input.