There's a "battle" volume setting! Is that what you want?
It lacks a slider in options, but you can change it through springsettings.cfg
https://springrts.com/wiki/Springsettings.cfg#snd_volbattleI assume adding more volume sliders to ingame menus would only require copypasting and adjusting some code in
LuaUI/Configs/epicmenu_conf.lua
and
LuaUI/Widgets/gui_epicmenu.lua
