No sound from Adobe's flash player

[Stefan Monnier <monnier@iro.umontreal.ca>]

I recently updated my Debian testing desktop (which moved it to Gnome 3)
and now the Adobe flash plugin doesn't give me any sound any more.
Luckily, Gnash gives me sound OK, but I'd still like to solve the issue
with the Adobe plugin for those sites that don't work with Gnash.
So, from what I understand the issue is that Adobe flash tries to output
the sound via ALSA but that doesn't work (hence the errors I get on
stderr "ALSA lib pcm_dmix.c:1018:(snd_pcm_dmix_open) unable to open
slave") because ALSA considers itself "busy with pulseaudio".

I'd love to get help here:
- how do I work around the problem so that Adobe's flash plugin gives
  me sound?
- why is ALSA's dmix refusing connections (after all, its name claims
  it's a mixer, so it should accept more than one input stream).
- Can I convince Gnome3 not to use pulseaudio?


        Stefan