Ok so in my current project I have my MIDI setup as follows:
 
I'm on track 4 and I have my Mobile Keys Keyboard selected as the MIDI input and my EMU sampler selected as the output.  I am on this track with no other tracks selected, and when I press keys on the Mobile keys keyboard I hear my EMU sampler (correct) with another synth on a completely different output (incorrect) both at the same time!
 
What gives?  This used to work properly with Sonar X1.
 
 

Input echo enabled on another track?

Hey FBB - thanks I never fully understood that function and it appears to solve the problem.  I thought for a moment I was experiencing the dreaded midi-cross talk problem I have heard about.

Thanks!

No worries. Input echo (think - send data to track synth) is automatically enabled on the in focus track and disabled when not in focus.

Over riding that and turning it on on a track not in focus will send any incoming data to that track's synth, hence you could hear it. Great if that's what you intended, annoying if you didn't.