So, you are using the Express both as your wireless router, and for Airtunes? I've never had much luck using it that way, seems that it only wants to work in one capacity, or the other. If you've had good luck in the past with this setup, than I've no idea why it would hiccup now. Try manually changing the channel of the Express, sometimes another wireless network nearby will be using the same channel, and cause some difficulty.
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