normally you can setup airport express as Airtunes-only... I never tried it since my AirportExpress is allso my Wireless router...
there is a possibility that you need to use AirportExpress on a different channel and need to log off from your other one to be able to use airtunes and then log on again on the other router to surf the net...
another solution (very silly one, not even 100% sure if it works but I guess it does, but maybe the way to go if you have more than one computer on the network connected to the router (maybe even wired)) Disable Wireless on your original router (thus iving the maybe wired computer(s) allso internet acces, connect your Airport express to the router... and use that as Wireless accespoint instead, giving both internet & airtunes access to the mac...
just a thought...
J
PS: just read your post again: Airport Extreme base station doesn't have Airtunes, only Airport Express...
PPS: so I think the solution: make sure you get an extra ethernetport available wich gives you internetaccess... connect Airport Express to it, set it up as Wireless Accesspoint & Airtunes... setup the Airport in your iBook... job done (should work...)
PPPS: my mistake... and you should disable the Wireless signal of your Linux Router...