Can I use Airport Express ONLY for streaming music?

I am going to try to assume your question. I hooked my Express to my Bose wave radio and it plays only the music from itunes (or streams from itunes) while everything else I do on my imac goes through the normal speakers. (I think that's what you are suggesting) I just came across this post because I want to see what else I can direct there (ie. Front row movie audio). If you set up up your airportExpress Correctly, You will see an added option at the bottom of your Itunes to select which speakers to play from. Let me know if If this was your question and if you need additional help.
-Craig

greenwavefan, you can definitely only use the streaming music feature and not using the router feature. I'd done this before when I had trouble setting up the AX router feature myself. When you setup your AX using AirPort Utility, choose "Connect to my current wireless network." Of course, you need to have your other wireless router network setup properly. The AX will then join that router's network, and you can stream your music wirelessly.