I have the Time Capsule and I'm very surprised to hear you're having problems. It's the simplest router I've ever used. Btw mine has never got as hot as you are describing.
I'm guessing you probably have a faulty one or maybe your speakers are causing interference?
I can use it fine. In fact, it's a pleasure to use and work with through Airport. The problems I'm having are it runs hot and drops it's wireless connection. The wireless could be down to the speakers. But there's still the heat problem from a completely sealed unit containing an HDD and router/wireless parts. It's idling now and last backed up 30 minutes ago, but the underside is still warm to the touch. It doesn't get stupidly hot unless I'm doing HUGE backups or wiping the drive. The longer the drive is spun up in the Time Capsule, the hotter it gets. And I doubt mine is unique in that respect. Some people might just have never bothered feeling the unit when it's running the initial backup or wiping the drive. Some might think the temperature is perfectly acceptable during those processes. But as I mentioned, with the amount of them that failed and the number of complaints on Apples website and the internet, I'd say it's a spectacular case of form over function and drives or internal parts where dying due to over heating. I mean, my router downstairs is vented and still gets very hot just being a router. I can't imagine the temperature it would peak at if it was sealed and I was to run a drive in it at full tilt for a hour or so while I did a huge backup as well.