Can I connect an airport express to my stereo, then connect the airport express wirelessly to a non apple wireless box so I can play music wirelessly from my laptop / iphone?
You won't be able to connect the express wirelessly with a non Apple wireless device in an "extend a wireless network" configuration. You would only be able to accomplish this function if you used a wired configuration, such as an Ethernet cable.
Yes. Please see the following Apple Support article for details.
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