Running qmmp somefiles.mp3 I get 3 warnings
VolumeALSA::setupMixer: Failed to find mixer element
VolumeALSA::setupMixer: Failed to find mixer element
VolumeALSA::setupMixer: Failed to find mixer element
and the volume slider is not functional. Using pavucontrol I can set the audio volume as expected, so I wonder if this is something to fix?
qmmp is version 2.2.5 on Debian 12. Audio is provided using pipewire, if I got this correctly. ps -ef doesn't list a pulseaudio job, but 2 jobs "pipewire" and "pipewire-pulse".
Select PipeWire or PulseAudio output plugin. (Ctrl+P -> Plugins -> Output)