I had the same problem.
It happened after I updated to the 18.104.22.168 kernel.
Upon rebooting was when the problem occurred.
What happened was my Alsa mixer volume control mysteriously
turned into some kind of Conexant mixer volume controller using OSS,
and Alsa was completely invisible with no way to select it.
Also sound only worked through headphones and I had to use the
mute/unmute trick to get both sides to work.
My solution was to boot to the 22.214.171.124 kernel; uninstall the
.16 kernel and lock the .15 kernel so I would not get any kernel updates.
My explanation is probably as clear as mud.
Oh and it's on a Acer laptop as well.
Hmm I wonder if this is why I experience bad sound quality?
Background: Bought computer in spring 2004 and installed Win98 (sigh)
plus MDK9.1. This is my first private computer so I didn't know what to
expect from an on-board sound system (VIA VT8235) but thought it
sounded too bad: with a metallic twang to all music. Somehow I figured
out that it should sound better and after much research found driver
Later on I migrated to PCLOS and never experienced an sound problems
there. Then I had a stupid accident resulting in a hosed HD (backed-up
only hours earlier!!) while trying to install PCLOS 2007. To cut it short:
I ended up trying out Linux Mint and has by now pretty much recovered
all data from the back-up. But, I have also realized I am reliving the
sound experience of MDK 9.1 and can't find my notes from back then.
Or, maybe it is the kernel thing mentioned above?
Yes, I just checked menu.lst: I am running 126.96.36.199. I will try out .15
and report back.
PS No, my computer is not an Acer laptop but a Shuttle SK41G box.