It’s possible but kinda crampy to open up and hack a volume button on the headphones.
I made something similar by hacking a cheap bluetooth remote camera shutter device.
It was the same technique - many ways to take over a simple switch - but easier because the bluetooth shutter thing was huge by comparison, easy to get into and less likely to be broken doing.
https://www.amazon.com/Wireless-Bluetooth-Smartphone-Compatible-Cellphone/dp/B073VSVTQF
Side benefit - since the headphones aren’t plugged in, you can hear the shutter sound…
a7