My friend and I have the MIT-100 on our bikes, we have the same GRMS radio as you (Midland 600GXT). We also both have Motorola Razr cell phones. Great minds think alike, I guess :-D
Anyhow, the music (he has XM, I just have an MP3 player for now) works great, and the GRMS radios work great with the "CB" cable that came with the intercom, and the PTT button works flawlessly.
However, we've both found that neither of our systems work correctly when our cell phones are hooked up. If the cell phone is connected, the PTT switch does not trigger the GRMS radio, although the cell phone works fine. As soon as the cell phones are disconnected, the PTT switch works, and the GRMS radio works fine.
So, anyway...the point of the story...to the original poster - if you also have a cell phone connected, try disconnecting your cell phone and see if that helps.
If anyone has any ideas on how to fix this, I'm all ears. The motorola razr has only a mini-usb plug, so we are using
this adapter between the cell phone and the intercom. I'm assuming it is an issue with the cell phone and I'm just SOL. Not really that big of a deal, as we bought the setups for music and GRMS...the cell phone useage was just an extra bonus. Still...a bit annoying that I paid $200 for this setup, it should work, and it doesnt.