AirTunes is the feature name for the RAOP protocol, which Apple somehow manages to break in subtle ways every time a new version of iTunes is released.

List of Stupid Fixes That Worked For Me:

Date Version(s) Description Notes
Dec 12 iTunes 9.0.2 plus an 802.11g AirPort Express Couldn’t get streaming to work reliably. Went into the Security preference pane, switched on the firewall and then switched it back off again. This is by far the stupidest fix ever, and I found it entirely at random somewhere else on the Internet.