I just had the same issue with my HD 4.50 BTNC (which should be the same). The little push buttons behind the volume controls were broken due to the impact. You can remove the earpads and unscrew the case to see it. I bought those push buttons from Amazon which fit perfectly: https://www.amazon.de/gp/product/B07PGZK4HR/ You need a hot air rework station, flux and a steady hand for soldering. You also have to be careful to not melt the case as you can't fully remove the circuit board from it. It worked for me and the headphones are like new now :-) Plus: I found a cable which wasn't properly soldered. Apparently it belonged to one of the noise cancelling microphones. After soldering it back on, I now actually have NC on those headphones. I always thought that this was just the worst NC I ever heard, but it actually was not active at all because it was broken right from the beginning XD