[Solved] Clip muting problem after speed-comping

Not sure what is going on there but you could just move the existing split point a little downstream to hear the tail of clip A thus moving the start of clip B accordingly. Just click drag left/right on the split. Does that help? - (Not with the 'bug' but with your workflow I meant.)

I may not totally understand what you want to do, but if you want to hear only A, you don't have to mute other clips. Select A, then use Shift+Space Bar. SONAR will play from the start of A to the end.
 
Speed comping is deceptive. It looks like it's easy, and it is on one level. But there are many nuances. One thing that's helped me is extending the Track Lane heights enough so that the various hot spots within the clip become suitably big targets.
 
Since you're transitioning from 8.5, let me toss in something I've noticed that may save you some time. Quite a few people make things more complicated than they need to be because they're not familiar with the workflow enhancements in the X-series programs. For example, there was one thread where someone had come up with this really convoluted way of cutting and pasting that didn't need to be that difficult.
 
Also, the more you can forget you knew 8.5 really well, the better . The people who get the most frustrated with the transition are those who want X3 to work like 8.5. It's a bummer there's a learning curve for a program you thought you already knew, but long-term, you'll save a lot of time and effort. 

Excellent answers.  Of course, shift+click.  I had it in my head that was only useable in the take lanes.  It's working great, in fact I'm also shortening the clip's loop length so I can really concentrate on manipulating just the end of the clip.  Most clips automatically crossfade well with the speed comping, but now and then I need to go in manually to slip back unwanted audio information.  It's working very well,  thank you both.