Alright you will have to retype that so that I can understand it. As I don't really know what the problem is. (new firmware and Mac OS doesn't work? Drivers for your stereo?)
Anway , I have set up an Airport Express before and I'll tell you what I can.
First you want to use the Airport Setup Guide (applications>Utilities folder). Going through that you can get your network up and going. Setting up a printer requires you plug the printer into the Airport Express USB port, and activate the printer from a remote computer.
As for using your stereo you need to plug cables (RCA line-out cable) into your stereo and then the mini jack part into the airport express. Select the port you plugged into on your stereo, and then with iTunes>preferences>advanced>general select Look for Remote Speakers. Activate this and close the preferences in the bottom right of the window use the pop-up menu to select your airport express and hit play.
Hope that helps. If not rephrase your question.