A few hours ago, I had a few problems with the set-up as in the subject line.
Firstly, I couldn't get COM2 to work properly, so I had to use one radio - not the easiest when you need about 4 frequencies set up.... Then, the engine volume slider in X-Plane's sound prefs seemed to have no effect. So, hearing the controllers when climbing was really difficult in my headset! Consequently, I was reducing power so much that I was stalling or close to it, and had to cruise at around 80 kts.

Then, I succeeded (while trying to change something with the mouse) in zooming into one instrument and opening another desktop! Oh dear!!!! I was In the middle of a practice ILS. I confessed to the controller that I was having a software problem, and that it was now more difficult than doing it for real, which caused us both to have a laugh. No harm done, and I'll have a try at the same flight tomorrow in FSX with A2A's Cherokee.
Someone in a forum said that 3rd party planes do not always obey X-P's sound prefs. I think some reconfiguring for this set-up will be needed. I'm using a USB headset/mic, but have external speakers connected to the iMac through it's headphone socket. If anyone else is using a similar set-up and can offer an easy (or obvious!) solution, I'll be grateful. The default new 172 was easier in PE from the sound point of view, and the Windows side seems fine too.