iPhone 4S + Universal Dock = ALL sound muted

I have several current-generation Universal Docks that I use just as a way to stand up my iPhone 4S while charging via USB AC adapter. (I have to use the Universal Dock because I keep a case on my phone, which doesn't fit with the actual iPhone dock). I've always been able to play music or use the speakerphone while on a phonecall while the phone was in the dock and charging.

Except something has happened recently where ALL iPhone sound from the speaker is muted as soon as the iPhone is placed in the dock. Even phonecall ringtones - they don't sound, the phone just vibrates.

It's almost like the iPhone thinks the dock is an iPod speaker dock or something. A second after docking, I see the volume overlay appears on the screen exactly the way it does on my iHome speaker dock - as if it's remembering the volume settings for this dock and binding to it. The USB cable DOESN'T EVEN HAVE TO BE CONNECTED! If I drop the phone into the dock wth NO cables connected, it still mutes all the sound!

So frustrating.

If you remove the dock from the equation, and just use the USB AC adapter to charge, the sound works just fine.

There are no software settings to alter this behavior. I have tried pairing an IR remote and fiddling with play/pause. I have powered off / rebooted the phone. The iPhone mute switch is not on. Volume is set all the way up. And I have searched these forums with no real answer.

Anyone know what's going on? Maybe it's an iOS 5.1 issue? I'm at a loss here.

I will see if this behavior is the same for all my docks - maybe it's just this one.