Avoid answer sound chaining after editor exit

I introduced a non-critical bug after altering how the reviewer exposes its audio to the Sound class, such that after exiting the editor of a card which was currently showing an answer, answer audio would get re-listed, and at the point the audio would replay, and do so double. Subsequent edits and exits from there would chain additions to that card, and play more and more copies. I added a check that answers have already been added.

Also, I extended this to the previewer, only because the function affected has exited there as a full copy from reviewer before and after my last changes. However, I believe reviewer only shows and exits, so it would not add audio multiple times, just live fleetingly and die. I don't know the specs of previewer (didn't even realize it existed til recently, and just looked at it for the first time), but I do see that it doesn't play the audio it embeds, either automatically or when clicking the audio icons.