the airport express light keeps flashing amber - all I want to do is to play music on my stereo through the computer - I already have a wifi at home

A persistent, flashing amber light indicates that any of the several potential conditions may require your attention:

• The base station has not yet been configured because it is new, or the reset switch has been set.
• You have selected a security configuration that is not recommended.
• One of several other conditions, such as a disconnected Ethernet cable or invalid IP address has been detected.

(ref: http://docs.info.apple.com/article.html?artnum=305101)

If you run the AirPort Utility, it will show an amber circle on the summary page next to the area(s) causing the problem. Click on this amber circle and it should provide an explanation. Please post back your results.

You might also take a look at this Apple document on how to setup the Airport Express for Airplay and iTunes.

http://support.apple.com/kb/HT4587