I have the exact same laptop with the same sound problem. I have not been able to fix this issue. I'm using Debian and hopefully alsa-1.0.17 will fix this problem. Although, originally when I installed debian, which shipped with linux-2.6.23, sound was working fine. If you're desperate, you can try installing the earlier kernel.
By the way, since we have the same laptop, have you been able to enable hardware acceleration of the Radeon Mobility HD 2600? Right now I'm using xserver-xorg radeonhd driver and it's playing videos too well. I know video is pretty much worthless without sound, but it's worth a shot...