Code:
Hostname ~ # rm -rf /lib/modules/$(uname -r)/alsa-driver
Some module files/etc... are protected and not removed during emerge -C, so you should run the above command to ensure you've a clean environment when switching from alsa-driver to in-kernel alsa. More reading:
http://www.gentoo.org/doc/en/alsa-guide.xml
You'll see this in the "Issues" section.
Edit - did you run alsamixer (emerge alsa-utils) and unmute your card? I don't mean to insult, so please disregard if you did, but it's a common mistake =)