I have problems with the sound on the Pi400 using HDMI. It was configured well and works in general. The Eizo display has built-in loudspeakers. When the sound is activated the display drives the audio from 0 to the selected loudness within 0.5-1s. But while watching videos the sound is randomly switched off from the pi400 and switched on again. Then the display repeats turning volume up from zero. The same happens, when I connect earphones to the display. With a notebook there was no audio problem at that display.
It seems to me it is getting worse when the Pi400 is short by CPU/RAM/IO. With low resolution videos I have fewer problems.
NeddySeaggoon had a nice hint which lowers the problem. The scripts plays a silent sound forever.
# Play silence forever so we don't miss the first few sec of sound.
aplay -c2 -r48000 -fS16_LE < /dev/zero &
# The kernel option does not seem to fix it