From your description it sounds like you're actually running an addon that's turning your sound volume up specifically for the queue, try disabling all of your addons and see if it's still too loud for you.
You can see
here what the interface is executing when the dungeon queue pops, there's nothing that should make it louder than the /run command I posted.
If you're still interested in the physical path to the sound file it's "sound/interface/levelup2.ogg"
You can also try
turning this into an addon, it may or may not work to mute the sound effect.
Lua Code:
local RestoreSound = function() SetCVar('Sound_EnableSFX', '1') end
hooksecurefunc('LFGDungeonReadyStatus_ResetReadyStates', function()
if GetCVar('Sound_EnableSFX') == '1' then
SetCVar('Sound_EnableSFX', '0')
C_Timer.After(0, RestoreSound)
end
end)