Really hard to say, you've already been far braver than I would in the same circumstances (I am such a coward re confrontation).
Personally, I'd buy earplugs, but I don't think that's a solution you'll go for
Check out the laws in your area re noise pollution I guess.
Then write her a letter stating the facts as you've laid them out above - you work from home, you understand the circumstances but this has become a longer term problem now, you don't want to take it any further but they need to come up with a solution.
That way they can't brush you off the way they did in the car. They're just being defensive, but the truth is, it isn't up to you to come up with a solution for them - this is their problem.
Good luck & keep us updated.