I have often found echoback to be a bit flakey. I have a project I'm working on ATM where it's stopped working. No matter what track I select, I hear EVERYTHING -- a cacophony of drums, bass and synths triggered all at once.
 
The global echoback setting on the tbar is on, and normally that works. Indeed, in other projects echoback is working fine. Anyone have a clue?
 
 

Is it anything to do with 'disable input monitoring' in playback and recording preferences?
 

It's called Input echo = for input monitoring

Enabling that setting did nothing. Even during playback I'm still triggering everything in the synth rack, regardless of what track is selected. It makes it impossible to work. And it's just on this one project, although as I said I've seen Input Echo behave flakey.

Could it be the Instrument/MIDI track inputs are set to Omni and one or more synths have "Enable MIDI Ouiput" turned on? Track input can be switched from None to Omni when Preferences > MIDI > Playback and Recording  "Always Echo Current MIDI Track" is enabled.
 
"Enable MIDI Output" is set in the "Insert Soft Synth Options" dialog, the VST2/3 Drop Down in the plug-in standard header and synth rack synth settings menu.

No that's not it either. "Enable MIDI Output" is off for all synths. And the "Always Echo Current MIDI Track" checkbox has no effect. 
 
Sometimes a project just gets screwed, and I have to copy paste every track over to a fresh project. I'm about ready to do that with this project but WHAT A PAIN!

I think 2 things should be established right off the bat:
 
1- Which version of sonar are you using ?
2- What audio interface are you using ?
 

OK working now. The combination that did it was enabling Always Echo Current MIDI Track in MIDI Playback and Recording preferences, DISABLING global Input Echo on the toolbar, and setting all instrument track Inputs to None. 
 
Nope, it's misbehaving again. Trillian (VSTi) is all I hear no matter what track is selected. *sigh* Sometimes I really hate Sonar.
 
I'm on latest Splat 2017.10. MIDI controller is a NI Komplete Kontrol. Audio interface is an Echo Firewire4.