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