I've been using Hifiberry OS with a DAC +DSP for about a year. I've never been able to use is as a music player. I've installed it, used the room correction features, and set up sources. I've used it as a DLNA renderer with BubbleUPNP, a LMS renderer with Squeezer and LMS, and an MPD source with YMPD and MALP.
My music library is on an NAS. It contains about 80K files. After many tries (and failures) I managed to get it to build an MPD database that shows content under the "music" source. I can right click on a tile and select either play or two flavors of queue. Play results in no output and nothing displayed on the now playing screen. I haven't been able to find a place that displays the queue.
Is the music player feature an experimental thing? When I use a different front end the current file shows on the now playing screen but the controls do nothing. I have had no success playing music except through the front ends mentioned above.