Streaming Itunes to Airport Express

I am able to stream itunes on Airport Express while also using my computer to access the internet via an Airport Extreme Network. I am also able to do the same at a vacation home which uses a Linksys network. However, when using the office network (small private office) with secured wireless access, which I have he access to, the itunes will only work on the wireless network if selected network is chosen on the banner bar at top of the desktop. The system sees both wireless networks (the Airport Express & the office wireless network). I was actually surprised that I could stream music to the express and still maintain a separate connection for internet simulteanously. I would like to be able to have the sam capability for the office as I do at home and the vacation home, any advice??