I found it difficult to get Alsa to work on Debian. The problem was that the OSS modules were loaded at boot time. The fix is to tell the hotplug system to ignore OSS modules.
apt-get install alsa-base alsa-oss cat /usr/share/doc/alsa-base/alsa-base.discover >> /etc/discover.conf
Reboot your system and check that the OSS modules are not loaded. Run
alsamixer to check that Alsa is running.