Runs a child process to play a second audio stream. Synced through named pipe. Recommend using with [cycle-adevice](https://gist.github.com/bitingsock/ad58ee5da560ecb922fa4a867ac0ecfd) to change the output device of the parent process.

audio-dupe.lua

--New child with start with the current 'aid' and 'audio-device' of the parent
-- non-Windows environments require the use of the 'socat' package
local platform_is_windows = (package.config:sub(1, 1) == "\\")
local options = require 'mp.options'
local o = {
    pipe_template = platform_is_windows and "\\\\.\\pipe\\mpvDupedAudio" or "~/mpvDupedAudio",      --windows format
    new_child_key = 'Ctrl+A',                           --Start a new child: 'Ctrl+Shift+a'
    cycle_child_control_key = 'Ctrl+Alt+A',             --Cycle child control: 'Ctrl+Shift+Alt+a'
    cycle_child_aid_key = 'Ctrl+Alt+a',                 --Cycle 'aid' of child: 'Ctrl+Alt+a'
    child_increase_volume_key = 'Ctrl+Alt+WHEEL_UP',     --Change volume of child: 'Ctrl+Alt+wheel'
    child_decrease_volume_key = 'Ctrl+Alt+WHEEL_DOWN',     --Change volume of child: 'Ctrl+Alt+wheel'
}
options.read_options(o)

local piper = platform_is_windows and " >" or " | socat - "
local mpvexe = "mpv" .. (platform_is_windows and ".com" or "")
local pipeInstance = {}
local control = 0

local function sync(prop, newVal)
    local timestamp = mp.get_property("audio-pts") or mp.get_property("time-pos")
    for k, v in pairs(pipeInstance) do
        if v then
            os.execute("echo no-osd set time-pos " .. timestamp .. piper .. o.pipe_template .. k)
            if prop == "speed" then os.execute("echo no-osd set speed " .. newVal .. piper .. o.pipe_template .. k) end
            if prop == "pause" then os.execute("echo no-osd set pause " .. newVal .. piper .. o.pipe_template .. k) end
        end
    end
end

local function cycle_aid()
    os.execute("echo cycle aid" .. piper .. o.pipe_template .. control)
end

local function switchControl()
    local lastInstance = 0
    for k, v in ipairs(pipeInstance) do
        if v then
            lastInstance = k
        end
    end
    if lastInstance == 0 then
        control = 0
        return
    end
    if control >= lastInstance then control = 0 end
    for k, v in ipairs(pipeInstance) do
        if k > control and v then
            control = k
            return
        end
    end
end

local controlling = false
local function quit()
    os.execute("echo quit" .. piper .. o.pipe_template .. control)
    pipeInstance[control] = false
    switchControl()
    print(control)
    if control == 0 then
        print(">>> Removed last child <<<")
        mp.remove_key_binding("quit")
        mp.remove_key_binding("cycle_aid")
        mp.remove_key_binding("volumeUP")
        mp.remove_key_binding("volumeDOWN")
        mp.unobserve_property(cycle_pause)
        mp.unobserve_property(syncSpeed)
        mp.unregister_event(sync)
        controlling = false
    else
        print(">>> Now controlling child: " .. control .. " <<<")
    end
end

local function start_child()
    table.insert(pipeInstance, true)
    control = #pipeInstance
    print(">>> Now controlling child: " .. control .. " <<<")
    mp.commandv("set", "msg-level", mp.get_script_name() .. "=warn")
    if controlling == false then
        mp.register_event("playback-restart", sync)
        mp.add_forced_key_binding("q", "quit", quit)
        mp.add_forced_key_binding(o.cycle_child_aid_key, "cycle_aid", cycle_aid)
        mp.add_forced_key_binding(o.child_increase_volume_key, "volumeUP", function() os.execute("echo add volume 5" .. piper .. o.pipe_template .. control) end)
        mp.add_forced_key_binding(o.child_decrease_volume_key, "volumeDOWN", function() os.execute("echo add volume -5" .. piper .. o.pipe_template .. control) end)
        mp.add_timeout(0.25, function()
            mp.observe_property("pause", "string", sync)
            mp.observe_property("speed", "string", sync)
        end)
        controlling = true
    end
    mp.command_native_async({
        name = "subprocess",
        args = {
            mpvexe,
            mp.get_property("path"),
            "--speed=" .. mp.get_property("speed"),
            "--pause=" .. mp.get_property("pause"),
            "--no-video",
            "--no-sub",
            "--osd-level=1",
            "--aid=" .. mp.get_property("aid"),
            "--audio-device=" .. mp.get_property("audio-device"),
            "--input-ipc-server=" .. o.pipe_template .. control,
            "--start=" .. (mp.get_property("audio-pts") or mp.get_property("time-pos"))
        },
        playback_only = false
    }, function() end)
    mp.add_timeout(0.25, function()
        mp.commandv("set", "msg-level", mp.get_script_name() .. "=status")
    end)
end

mp.add_forced_key_binding(o.cycle_child_control_key, "switchControl", function()
    switchControl()
    if control == 0 then
        print(">>> No active children <<<")
    else
        print(">>> Now controlling child: " .. control .. " <<<")
    end
end)

mp.add_forced_key_binding(o.new_child_key, "start", start_child)

mp.register_event("shutdown", function()
    for k, _ in pairs(pipeInstance) do
        os.execute("echo quit" .. piper .. o.pipe_template .. k)
    end
end)

As for the flickering issues (which I haven't been able to reproduce) you can try specifying the command to spawn in a minimized window to see if it helps. as a test, try replacing line 85:
mp.add_forced_key_binding(o.child_increase_volume_key, "volumeUP", function() os.execute("echo add volume 5" .. piper .. o.pipe_template .. control) end)
with
mp.add_forced_key_binding(o.child_increase_volume_key, "volumeUP", function() os.execute('START /MIN CMD.EXE /C "echo add volume 5' .. piper .. o.pipe_template .. control .. '"') end)
then see if volume up (default: Ctrl+Alt+WHEEL_UP) produces the issue.

---

as for the delay issue, I can only assume your systems are under heavy load. I ran some stressed scenarios and it was a bit out of step but nowhere near 3 seconds. Unfortunately, the only real fix without dependencies would only result in 1 second latency at best as lua's os.time() only produces seconds.

Another possibility is Windows Defender (or some other AV) is analyzing system calls from mpv and delaying it.