There is a confirmed problem with Mac OS X 10.6.8’s audio output, specifically where digital audio output stops working or intermittently stutters after updating to 10.6.8. This has been reported to affect both optical and HDMI audio output, however optical seems to be more prone to experience this issue.
Some users have reported they’ve fixed this problem by replacing the AppleHDA.kext kernel extension file from a Mac running 10.6.7, but that workaround doesn’t seem to help everyone.
Rumor has it this problem also exist in the latest Golden Master of Lion, and Apple has acknowledged they are aware of the issue and it is being investigated by engineering. More information can be found here: https://discussions.apple.com/thread/3139275
Until Apple addresses this problem, it is not recommended that you updated to Mac OS X 10.6.8 if you rely on outputting digital audio.