I have a little problem getting my songs from my Macintosh to play via my Airport Express AirTunes enabled base station. Help greatly appreciated !

The situation is simple: I have two macs and two base stations. BaseStation-1 is a DSL modem and provides internet access, it is connected wirelessly to Mac-A. BaseStation-2 is the Airport express station and is hooked up to my HiFi as well as being wirlessly connected to Mac-B. However, BaseStation-2 and Mac-B are too far away to join the same WiFi network that BaseStation-1 creates. Mac-A routes internet access to Mac-B thru an ethernet cable.

Unfortunately only iTunes on Mac-B sees the AirTunes output in the little lower-right menu. And since my music is all on Mac-A, I was wondering if there was a way of making it see the airport express station.. it is only one hop away.. but TCP/IP networking is so complex...