Music playback sound level goes up after 3 seconds

iMac 24 in M1, OS 11.5.2.

In the Music App, the Playback sound goes up after 3 seconds or so when a track is begun after a period of no playbacks. If I select next track whilst one is playing, there is no change in sound level.

I've switched sound check on and off, but this doesn't change this behaviour.

Also the balance across the sound frequencies changes markedly between low and high volume.