I have Flysimware's MU-2B and will test it with KLGA (default FSX).
Just tested it, it works fine:
The Collins COM in the MU-2B is COM2. If you want to use COM2, you need to go to "Options #2 -> Advanced Options" and enable "Use COM2 Radio".
(and you have to enter 121.88, as it jumps from 121.85 to 121.88)
COM1 is in the GPS unit and is controlled separately.
I like to use the Collins, too, that's why I asked Dave to add support for COM2 to PF3
And I already sent him something for the wish list: Use FSUIPC offset 0x3122 to determine the COM selected in the audio panel
