I picked up this bluetooth set back in April for our 4 day trip to HMT and I was very happy with it. I used it to communicate with my daughter riding with my father in his car mostly, but there were times my daughter rode in the seat next to me in my car and it worked very well in both cases. Two days we used them for 8+ hour rides. As long as we were in range, we could easily communicate with each other. It supports up to 8 units in an open intercom configuration (no push to talk). Take a look, I think it would meet your requirements.
Fodsports Motorcycle Bluetooth Intercom
Cordless - yes, bluetooth from unit to unit, not dependent on cell phone
Ability to listen to music from my phone - yes, switch between intercom and cell phone with the push of a button
Ability to talk to passengers but really not other machines - it communicates from helmet to helmet, if they are in the same car or another car
Ability to easily turning and off the mikes so I don't have to listen to the passenger singing along with the music. - can turn off the mic, but you can switch from intercom to cell phone with the push of a button
Separate volume controls - yep, each connection has its own volume setting, though I admit its a bit tricky with knowing what the volume is set at
Waterproof - yes
Affordable but not a cheap piece of crap - I bought 2 units for $150
Earpad and mic installation is key, get them in a good position to pickup your voice and hear the others.