buffer underrun in Playback-Qt

I think I broke something when I ported Playback-Qt to Qt 6. Running "Test" in the playback settings dialog works fine for me, also playing some files works flawlessly. Other times I get buffer underruns, especially with a larger file I have (~450 MiB WAV). ALSA and PulseAudio play it fine.

I've tried to debug, but haven't figured it out yet. I'm opening this issue in case someone else has an idea how to trace this down.

I'll be gone for a week or so, but hope to get back to debugging it soon.