Yes, everything is understandable etc., but if someone already has no desire to use their garden at 45 dB and does not want to have their own music (which was discussed a few posts earlier) and is already aware that there is probably a hypersensitivity. Then what’s the use of beating around the bush? Nothing physical will help there. I advocate for exposure and occupation with something positive. So go out there and do something you enjoy and that requires concentration. This will direct the focus there. At first only for a very short time, then longer and longer. Eventually, a habituation effect will occur. It must, otherwise you simply cannot use the garden. If necessary, get help for that.
The neighbor will also not agree to never listen to any music anymore or to play it so quietly that they themselves can no longer hear it in their garden. I then also wonder, are there no kids in the neighborhood who are loud, lawn mowers etc. All of that does not bother then? To me, all of this speaks for a pathological over-focusing on this one supposed disturbance.
And I don’t want to say with that, don’t be so sensitive – the experience will be real and terrible. But there are effective methods for that too, which are not hedges or gabions.