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.