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:
| 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.