Tony, I have the same problem with the ACL888b.
After read a lot in this forum and others, and tried several installations I decided to write you my experience that may be helps to find a solution.
My first installation was made with old bootCD method, then updated up to 10.6.3 and solved the audio issue with gigabeast without any problems.
Then I decided to do a fresh install with iboot + multibeast and the audio issue never can be solved, even in 10.6.3 update. (I used DSDT for UD2 F10), but then I tried every method I read and off course I've rebuild caches, repair permissions, rolled back 10.6.2 appleHDA.kext, installed voodoo, deleted HDAenabler, downgrade the bios to F8, and many many other things with no good results.
So, I think the problem is not the 10.6.4 update, I think the problem is when you use iboot+multibeast together (even in 10.6.3 update)... I don't know, may be some DSDT entry causes the problem.
Thanks Tony!!!!!
GA-P55-UD2 F10 - i5 - GT 240