For those new to comping (like me), here's a hint that might be helpful.   In Edit > Preferences > Project > Record under "Auto Punch", the default seems to be "Mute Previous Takes".    Therefore when you have Comping on (in the same menu), playback will default to playing only the new take lane and muting the old one.   The muted clip waveform will show up as a "hollow envelope" and "Clip Mute/Unmute" in the menu will not unmute it (nor will the "K" key on the keyboard).   
 
The hint is this:  Before recording using punch-in with comping on, make sure the "Mute Previous Takes" box in the Record menu (above) is NOT checked.    Then, next time you playback, you can mute/unmute clips in the take lanes using either the (right click) menu or the "K" key.   
 
Maybe someone can suggest a workaround if you've already recorded with the "Mute Previous Takes" box checked.   If there is one I've not yet found it after lots of trying.
 
I am using SONAR Professional.   I do not know if issue is the same in other versions.
 
SOLUTION FOUND:   Scroll down in this thread and read my next comment regarding solution found at
"http://forum.cakewalk.com/Un-Mute-punch-in-SOLVED-m3331262.aspx"

Thanks for posting this.

Nice. A useful contribution indeed.

What if you already recorded the items with Mute Previous Takes?  How does one go about muting and unmuting the takes?  I am running into this now.  It just assumes I want the last one no matter what.  I can't seem to audition previous takes?  Is there any way to correct takes already made with the Mute Previous on?  I thought this meant at playback...

Pat, you are having exactly the same problem I had.   I couldn't figure out how to un-mute either and had to re-record. 
 
However, since the original post, I FOUND A SOLUTION in another thread.   A very knowledgeable user named "brundlefly" gives the solution in his answer to DeeringAmps' post called "Un Mute punch in SOLVED!".   This is at
"http://forum.cakewalk.com/Un-Mute-punch-in-SOLVED-m3331262.aspx"
 
You should read that post, but to summarize it, there is a hidden mute function (have to right-click the "Erase" button in one of the menu bars at the top (to the right of the Move, Edit, and Draw buttons) and change it to "Mute".   Then you just swipe the mouse over the portion of the muted waveform that you want to un-mute.
 
THANK YOU BRUNDLEFLY! - I would have never found this on my own.
 
 IMPORTANT UPDATE:  This is a little trickier than I first thought so I am adding this update.    When swiping, a swipe above the waveform un-mutes, a swipe below the waveform mutes.  

I think it defaults to muting the clips for a good reason. I remember watching a Cakewalk comping video in which when you start assembling your final track from all your takes, the clips would mute and unmute automatically allowing you to select only the sections you wanted to use. I can't remember for sure, but it seemed to make sense. I've never really gotten the hang of it, so I may be confused. Wouldn't be the first time. 

Here it is: https://www.youtube.com/watch?v=TPi1BkBhEvE
 At around minute 3:00 it explains what I was talking about.

dcbarton5
THANK YOU BRUNDLEFLY! - I would have never found this on my own.

You're welcome. Good job hunting up the old thread. Glad you could get some more mileage out of it.

+1 on the Thanks brundlefly!