Apple’s latest Snow Leopard update seems to be wreaking havoc with audio out through the optical and HDMI ports on the Mac mini.
OS X 10.6.8 Causes Mac Mini Media Centers To Fritz Out

Apple’s latest Snow Leopard update seems to be wreaking havoc with audio out through the optical and HDMI ports on the Mac mini.
The latest major patch update to Mac OS X 10.6.8 just got pumped through Software Update, and this is one patch everyone with a Mac is going to want to make as soon as possible: without it, you won’t be able to upgrade to 10.7 Lion when it is released on the Mac App Store next month.