Could be either that you don't have alsa (which would be odd, but you could check by popping open synaptic and seeing if it's listed under installed packages) or that it's there, but not started, in which case try

alsa_init

I've seen Stephen (who knows a lot more than me) refer to (IIRC)

alsa autoconfig

but I've not had to use that post 3.2 (I'm not sure it's in 3.3).

You'll probably have to do some tinkering with Kde's Control Centre after starting Alsa to get the system sounds working again, but this is the least of your problems and Alsa really is where it's at.