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I am not sure about the USB speakers, but you can read customer reviews of the Airport Express on Amazon. Perhaps someone there has an idea that might be of use regarding the USB speakers.
It seems odd to me that you can't easily use the USB speakers though. Maybe Apple needs to fix the Airport Express to allow this.
It's basically only to connect a printer so I guess you can't: http://store.apple.com/us/question/answers/readonly/my-speakers-have-only-a-usb-connection-will-they-work-with-airport-express-ae-or-does-ae-only-work-with-speakers-that-have-miniplugs/QK2DJ9JK2PCJ4FP4K