do you have the required module installed?

afaik gain control sysfs is not a part of vanilla linux kernel. so you may need to port some patch from elementalx kernel, rather than just simply enable some kernel config

there is a app for this https://github.com/TigerBeanst/TigerInTheWall

have you tried edit /system/vendor/etc/mixer_paths.xml file ? it may include volume and gain config

maybe you can try rclone . it support saf and streaming. so probably is what you want

im already using archlinux arm on my phone for some years. there is really nothing i need to degoogle at now. the only google product im using is gmail (3rd part client) and play games (run in waydroid).