soens
The Process menu is for Audio tracks. I believe if all you have is midi tracks controlling outboard synths, a bus fade won't work because the synths are playing thru your interface, which is also outside Sonar. For Sonar to manipulate the synth outputs they need to be recorded into Sonar on an audio track and sent to the Master Bus with an automation envelope.
Or you can apply cc11 controllers to the end of all your midi tracks as mentioned above.
The synths need to be fed back into Sonar, to an audio track. You could then route those to a bus with an automated fader on it.