I upgraded to debian squeeze some time ago. In debian lenny I sometimes had problems with the sound card not being found.
On lenny I used alsaconf to rescan the cards and reload the modules needed for them.
It seems now that the alsaconf script isn't included anymore in the alsa-utils package. At first all seemed ok in squeeze, I always had sound. But today, the sound card wasn't detected.

I have an Intel onboard card:
Audio device: Intel Corporation 82801I (ICH9 Family) HD Audio Controller

So how do we fix this? Not that hard it seems, just force the reload of the sound modules:

debian:/# alsa force-reload
Terminating processes: 3435.
Unloading ALSA sound driver modules: snd-seq-dummy snd-seq-oss snd-seq-midi snd-seq-midi-event snd-seq snd-pcm-oss snd-mixer-oss snd-hda-intel snd-usb-audio snd-pcm snd-usb-lib snd-rawmidi snd-hwdep snd-timer snd-seq-device snd-page-alloc.
Loading ALSA sound driver modules: snd-hda-intel snd-usb-audio snd-seq-dummy snd-pcm-oss snd-mixer-oss snd-seq-oss snd-pcm snd-usb-lib snd-hwdep snd-seq-midi snd-rawmidi snd-seq-midi-event snd-seq snd-timer snd-seq-device snd-page-allocWARNING: All config files need .conf: /etc/modprobe.d/nvidia-kernel-nkc, it will be ignored in a future release.

This will also kill any programs locking the sound modules (in my case on KDE is was kmix).

KDE detected the change and switched to the right sound card. It was using my webcam as sound card before.