I return with (mostly) good news!
Just did a flight from Heathrow to Dublin and the sound issue I had a month earlier didn't occur, even when I was switching screens and everything. My only issue was that right in the middle, FSX decided to crash. I'm /pretty/ sure this was the J41's fault, I'm having a lot of issues with it. The IAS hold makes the plane go out of control and FSX almost always crashes...
I think I might have accidentally touched the "Headset" button a bunch of times or there was an issue with the sound driver that it decided to use and it just got confused. I touched neither and it worked perfectly. Appart from the crash.