Hello,
 
I am experiencing something weird with the bracket keys.   I believe they are unassigned by default, but I just checked and mine are set to unassigned, yet they seem to act as a mute/unmute of a track that I previous had muted.  
 
These are the 2 buttons to the right of the "P" button.   
 
Is this expected behavior?
 
Thank You!

That is kind of weird . . . because as far back as I can remember, the two bracket keys, though unassigned as Sonar keyboard shortcuts, have always been useful for bumping the pitch up/down an octave, or velocity values up/down by 10 in the "event inspector" and "note properties" fields  . . . at least that how I've always used them.
 
I can also verify that after you mute a track, the bracket keys also act as a keyboard shortcut to mute/unmute the tracks again . . . hmmm, is this new, or did we just never notice it before ?

Interesting find.
 
I attempted to confirm the reported bracket behavior in the SONAR Platinum, SONAR X3, SONAR X2, SONAR X1, SONAR 8.5, and SONAR 4 (Yes, I still have my SONAR 4 and Pro Audio 9 disks ).
 
All versions of SONAR between Platinum and version 8.5 exhibited the reported toggling behavior.  Only SONAR 4 did not contain the toggling behavior.  So this undocumented behavior of the bracket keys was added somewhere between SONAR 4 and SONAR 8.5.
 
It's kind of a neat "feature", because the last Mute, Solo, or Record button enabled can be toggled by the left and right bracket keys.
 
Congratulations!  You may have stumbled upon an "Easter Egg".  If you don't know the meaning of "Easter egg", then look it up here: https://en.wikipedia.org/wiki/Easter_egg_(media).
 
Hope this helps,
-- Ron

[ and ]  work on many of the track view and inspector controls, but they only work on the currently-selected control (the one with the white corners). Either click on the control or use left/right arrow keys to change the selected control. They work on the volume/pan/gain controls, but only move in jumps that may or may not be useful. Pan moves by about 7% in track view and 10% in the inspector. Interleave, phase and input echo also respond, as do all controls in a Send.
 
Cheers, Peter.