I found this in response to a question on the i'net ... not sure if the directions are correct, but no doubt Diki can tell you ...

Im not really sure if anyone here makes one currently, audiopile might be able to rig one up for you at a decent price, not sure if you count him as a forumite though.

I can tell you how to build one though. You should be able to get parts from any radio shack or a multitude of online dealers. Just get a project box and mount a female and male xlr in it. connect the ground (pin 1) on both connectors. also connect audio + and - (pins 2 and 3) through to both connectors. at this point you basically have a mic cable in a box. Now wire another pair of wires coming from + and - of one of the connectors. Connect these wires to a momentary normally open push button switch. When you press the button + and - will be connected and this is what does the muting.

You should be able to do this pretty easily with basic soldering skills, you can probably dig up some screw terminal xlrs and a crimp connector push button switch otherwise. Or just buy one, your call.

Here's a link to the site, which also offers some other suggestions ...
http://acapella.harmony-central.com/archive/index.php/t-1510736.html