System Preferences > Users and Groups > Login items and remove
iTunes Helper from the list. Then, log out and log back in. Try connecting your speakers.
**Edit: The iTuneshelper.app is part of the iTunes package. To diable the iTuneshelper.app you have to view the contents of the iTunes.app by Option clicking, or right clicking, and choosing "Show Package Contents". The iTuneshelper.app is located in the MacOS folder.
Rename iTuneshelper.app to iTuneshelper-disable.app and this will prevent it from launching and being added to Login Items.
This method tested and working with OS 10.8.3 and iTunes 11.0.2.
-From useer GageGecko on the macrumors forum.
Note that this will keep it from adding itself to the login items whenever you open itunes, but will also mean you have to open itunes yourself when you plug in your phone, ipod, etc.