Yes, thank you - my misunderstanding of how the name Sid works but I could not at the time remember that it's Squeeze and Wheezy.
What I can get from Debian sites including the ones you link to, is confusing. They give ALSA as the Linux standard and show it included with Lenny but OSS4 as the FreeBSD standard of which they say Squeeze contains a technical preview
and list it and not ALSA as part of Squeeze. The wording is a bit ambiguous as to whether these include the actual card drivers or are front ends to a lower level. Alsactl init
(?) couldn't find any card at all. According to Windows it is Sigma Tel Audio.
There was an extensive discussion about losing sound in a pure Squeeze system two years ago but it looks as if that problem was a changed mixer default. At present, the mixer is set to Dummy with no options.
I wonder if there is an ALSA-OSS clash or a boot sequence setting changed to default without non-free drivers. You have at least given me some directions to poke around in, which has given an idea of what sort of problem might be at the heart of it.