remix surround channel levels

channel mixer.lua

cmCenter = 1
local defCenter = 1
cmFront = 0.707
local defFront = 0.707
cmSide = 0.707
local defSide = 0.707
cmBack = 0.707
local defBack = 0.707
cmLFE = 0
local defLFE = 0

local function mix(channel, adjustment)
	if channel == "reset" then
		cmCenter = defCenter
		cmFront = defFront
		cmSide = defSide
		cmBack = defBack
		cmLFE = defLFE
	else
		_G[channel] = _G[channel] + adjustment
		if _G[channel] < 0.001 then _G[channel] = 0 end
	end
	mp.set_property("af", "lavfi=[pan=stereo|FL="..cmCenter.."FC+"..cmFront.."FL+"..cmSide.."SL+"..cmBack.."BL+"..cmLFE.."LFE|FR="..cmCenter.."FC+"..cmFront.."FR+"..cmSide.."SR+"..cmBack.."BR+"..cmLFE.."LFE]")
	mp.osd_message("lavfi=[pan=stereo|\nFL="..cmCenter.."FC+"..cmFront.."FL+"..cmSide.."SL+"..cmBack.."BL+"..cmLFE.."LFE|\nFR="..cmCenter.."FC+"..cmFront.."FR+"..cmSide.."SR+"..cmBack.."BR+"..cmLFE.."LFE]", 5)
end

mp.add_key_binding("Ctrl+Shift+F8", "mReset", function() mix("reset") end)

mp.add_key_binding("Shift+F8", "cUp", function() mix("cmCenter",0.1) end)
mp.add_key_binding("Ctrl+F8", "cDown", function() mix("cmCenter",-0.1) end)

mp.add_key_binding("Shift+F9", "fUp", function() mix("cmFront",0.1) end)
mp.add_key_binding("Ctrl+F9", "fDown", function() mix("cmFront",-0.1) end)

mp.add_key_binding("Shift+F10", "sUp", function() mix("cmSide",0.1) end)
mp.add_key_binding("Ctrl+F10", "sDown", function() mix("cmSide",-0.1) end)

mp.add_key_binding("Shift+F11", "bUp", function() mix("cmBack",0.1) end)
mp.add_key_binding("Ctrl+F11", "bDown", function() mix("cmBack",-0.1) end)

mp.add_key_binding("Shift+F12", "lUp", function() mix("cmLFE",0.1) end)
mp.add_key_binding("Ctrl+F12", "lDown", function() mix("cmLFE",-0.1) end)

Using a modifier (Shift or Ctrl) and the function keys (F8-F12) you can adjust the mixing level of surround channels on the fly
Ctrl+Shift+F8 resets the levels to ffmpeg defaults

F8 adjusts the center channel
F9 adjusts the front channel
F10 adjusts the side channel
F11 adjusts the back channel
F12 adjusts the LFE channel