It's the same on my raspberry Pi 4 4GB with MATE 20.04 armhf.
I've two monitors working and therefor I have to commend the line hdmi_drive=2 in config.txt because else the monitors wouldn't work.
I get the same error running "sudo amixer cset numid=3 1" as linux203.
And it works on RaspberryOS as well.
any suggestions?