I finally got it working. I think that what made the trick was mapping or exposing the alsa.conf file, I followed the tip of this comment.
But I also updated the base to core22, removed some interfaces already provided by the gnome plugin and also all the libraries in the stage-packages, leaving only those for audio.
One last thing to mention: I could not debug the mapping of the alsa.conf file with --devmode, because I could not find where it become installed or get a shell into the snap to see if the mapping occurred. I had to release it, and install from the store, to be able to test it. I do not know if there is a better way to do this.