Setup: Dell S3221QS over DisplayPort, audio goes to the monitor from the GPU. KDE's volume keys only touched the PipeWire sink, which is digital attenuation applied before the signal ever leaves the PC. The monitor's own amp gain (DDC/CI VCP 0x62) sat wherever the OSD left it, so pressing volume down did way less than expected.
This daemon listens for kmix's media key events on D-Bus and copies the sink level to the monitor via ddcutil after every press. Plasma keeps ownership of