Additional information that might be interesting that I've picked up from reading the comments thread on Amigaworld.net:
As stated, the problem was something to do with initializing parts of the southbridge and that normally this is done in the BIOS. Apparently, the next update to UBoot will do just that. The current driver performs that initialization manually.
Based on this information, I'd guess that onboard sound will also work under Linux once the new UBoot is released...?
I'm quite excited about this, as you might be able to tell :-)