One of the known issues with Ubuntu on a Lenovo x300 is the sound (up to and including Ubuntu 8.04). There are a few links out there that point to solutions. The one that worked for me is this one in Mikko's Blog. One very important note: Check which version of the kernel you're running before you start.
uname --release
Then substitute your kernel version in the rm command. I also got a bit confused by the statement under the "Sound" heading that said to remove the old sound modules. I eventually figured out that he must have been referring to the rm command in his script, rather than actually removing the running modules.

FYI: My rather short list of Ubuntu and Lenovo x300 links is here.