lsblk listed two devices. Therefore, I was wrong. I was expecting to
have the boot Micro SD Card detected as sda and the USB stick as sdb!

Nevertheless, this is not the end of my troubles trying to make my Pi
3B+ see reason and play sound files. Alsamixer is only showing one
control, PCM. I am struggling to make it display all controls not just
one. It seems the Pi image is heavily infected with pulseaudio,
something I want to rid myself of. I uninstalled all packages with a
'pulse' in their name but still alsamixer obstinately shows only one
control, PCM.