It worked out only temporarily unfortunately. The contents of the /mnt/mpd folder disappeared quite soon after they appeared. And they stayed disappeared for several days, even with rebooting the Pi and desktop and router. Then yesterday they appeared again and I was able to play some of the tracks. But now gone again. The radio playing was also on/off — sometimes just stopping for no apparent reason after playing for an hour or more.
So I decided to try your suggestion of the Squeezebox Server:
„Just install the Server with one click (select 7.9 nightly) and activate Squeezelite with the sysdefault hifiberry amp option selected (make sure MPD is not running anymore). Then, you can control the playback, volume, etc. via the Squeezebox Server web interface or with one of the great Apps for Android (Squeezer) or iOS (iPeng).“
You didn’t say how to activate Squeezelite, but I found it in the Audioplayer. I have turned off the MPD player and ympd. Now one of my radio stations appears to be playing — I see it in the Logitech Media Player, and in the Squeeze App on my phone. But no sound once more! Is there another setting somewhere that I have missed?