Phone keeps setting itself to silent mode?

I keep having this mysterious problem with my Droid where I miss all my calls... then I go into my call settings and find that my ringtone is set to "Silent"... so I reset it to the Muppet Show Theme as any rational person would... and then a few days later I start msising all my calls again and whaddaya know? It's set back to "Silent"... what is making this happen?

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