Well I do know a Railsounds 6 board won't work with a Legacy R4LC. I suspect if you find a Railsounds 6 board it would have to be driven with an RCMC. Sounds like a RCDR wouldn't work either. I haven't come across too many engines with RS6 boards anyway.
Pete, you know something that Lionel doesn't know! I have the Legacy Lionmaster T1 Duplex, it has a RailSounds 6 board and the Legacy RCDR. That's the tender I was doing the experiments with.
While it works with the RCDR, it doesn't work with the RCMC equipped locomotives, I tried a few. However, I am able to take the T1 Duplex tender and using a Legacy R4LC-C03 in my test board feeding the optical link to the tender, I can drive it and get all the sounds properly rendered.
I don't pretend to know all the in's and out's of the compatibility, much of this stuff is discovered by tinkering with various combinations.