Actually, this appears to be standard behaviour across the board in Linux.
I run mostly Puppy, and occasionally AntiX. I've got a 15-yr old Dell Inspiron lappie, where the headphone socket is completely worn-out. Since my soldering skills are, um, shall we say 'non-existent'
, I decided to try the external sound card route, since I have available USB ports, and that would give me new headphone
and microphone sockets.
The sound card works fine, but I, too, had to go into 'alsamixer' and unmute it first, followed by fine-tuning the remaining settings. Pup doesn't use PulseAudio, incidentally, so ALSA is standard fare anyway.....but you still need to select the correct card before the ball starts rolling!
So, it appears this business of 'automuting' would seem to be standard implementation in Linux, when using anything
other than the built-in, originally-configured hardware.
Mike.