airplay for one of my two airports stopped working

Hello, I have a wireless network in my apartment that includes the latest generations of an airport extreme (which is connected to my cable modem), two airport expresses and a time capsule. The airport expresses are used as signal amplifiers and as airplay devices (each one is connected with an audio cable to a stereo).

For some reason, airplay on one of my airport expresses stopped working (no sound at all). Airport utility says that it is on line and that the connection is excellent, and I am able to play other components on the stereo that it's connected to. When I choose the other airport for airplay in System Prefs, it works fine (and it is much further away from my laptop, and behind a wall to boot, while the one that isn't working is only about 3 feel from my laptop, in line of sight).

I bought the airport express that isn't working only 6 months ago, new, from an Apple Store. It has worked fine until now. Does anyone have any ideas?

Thanks.