No sound on 12.10 installation on Acer Revo - how to get some?

[Chris Green]

I am having trouble getting any sound output on an Acer Revo R3600.

I first tried after upgrading an Ubuntu Server installation to 12.10,
that produced nothing.

I have now done a completely fresh install of Lubuntu core and that is
just as silent.

Everything *appears* to work, e.g. if I run alsamixer on the command
line I get all the expected bits and can turn the gain up on 'master',
'phones' and 'speaker' but still I get nothing out.  

If I run the default rhythmbox it thinks everything is OK and 'plays'
but no sound comes from the speakers (I'm using the headphone output
driving an amplifier).  Jiggling the connections produces a crackle from
the speakers so they are connected and live.

Can anyone suggest what might be wrong?

-- 
Chris Green