Pirate Audio set volume without mopidy

I am writing a C program that connects to an MQTT broker and plays an mp3 when requested via MQTT.
I am using it with fhem, a home automation server, for signaling various things, like my doorbell or incoming phone calls.
I tried to use the 2 MQTT plugins for mopidy that I found on github, but both repositories that I found had errors (that I think I fixed) but mopidy still stops working after a while. I now have a working C program for the mp3 playback, but somehow I can’t set the volume of the pirate audio. Without starting and stopping mopidy I get no sound after reboot.
I am not very fond of python and would like to get everything working in C. Apart from the volume control,
the next step would be to use the st7789 as well.
Setting the volume at startup would be ok, but I don’t want mopidy to run. Any ideas?