Do you want 9 and 13 to play together? Or are you trying to comp the parts?
It looks like you split 9 on the measure before the mute. Split it at the point of overlap with 13.
Split 9 again where 13 starts, and split 13 where 9 ends. Now you should have all individual clips with no overlap. You should be able to mute/un-mute with K any of the individual clips.
I think you have to split wherever an overlap occurs in order to un-mute correctly.

Thanks for sticking with me, John. I appreciate that you're trying to help, but I don't think we're getting anywhere. Like I said, I've tried splitting 9 before, after and dead on the overlap with 13, to no avail. I couldn't see the point in splitting 13 where 9 ends because the whole of 13 is audible, but I gave it a try anyway. It has no effect on any part of 9, and both parts of 13 remain audible, just as they were before I split it. Hell, I've even tried cloning the entire track and deleting all but one ghost layer (i.e. such that it is the only audio in the whole of that cloned track) to see if it became audible, but still nothing.
 
"I think you have to split wherever an overlap occurs in order to un-mute correctly." Seriously? If you record a bunch of separate takes in layers on the same track, you have to go back and split them all wherever an overlap occurs, otherwise you can't pick what to hear and what not to hear using a quick and simple mute/solo command on each layer? What an unhelpful feature!
 
I think I've just been unlucky and used a combination of settings that the software designers didn't anticipate. I'm going to investigate further using different settings, and maybe try loop recording rather than stopping between takes with auto-punch enabled. If I discover anything worth reporting, I'll come back and provide an update. Thanks to all who contributed.
 
A belated thought: I just took a look at the Help file page on Loop Recording (better late than never ) before clicking 'Submit', and while the 'Mute Previous Tracks' box is checked in the settings shown, the 'Auto Punch' box is not. It has only now occurred to me that there wasn't any actual need for me to use auto punch, as I was recording on a new, clean track with no other audio in danger of being erased; loop recording would have done the job. It's just that immediately prior to this issue arising, I'd been auto-punching my friend in and out on another track to fix a small mistake, so my brain was in auto-punch mode.
 
It would probably be fair for SONAR's developers to say, "Why would you want to do what you were doing, the way you were doing it? There's no need!", but I still don't think any combination of settings should result in what I've had happen. Perhaps there's a way for them to prevent you selecting that particular combination of settings.

The reason I suggested splitting 13 was I believe you had "mute overlaps" selected in preferences. Since 13 was recorded after 9 you would hear all of 13 but 9 would mute at the overlap. After you split the clips at the overlap points you should have all separate clips which you should be able to toggle with "K" to mute or un-mute any individual clip. Don't confuse the mute button which mutes the entire track or layer. K should work on any selected clip.
I ran into this just this past week, but I was punching in so all the takes were the same length (no overlap).
I will try to re-create your issue by recording different takes that do overlap.
Hang in there.