Program.cs

using System.Runtime.InteropServices;
using NAudio.CoreAudioApi;
using NAudio.CoreAudioApi.Interfaces;

namespace PauseOnDisconnect
{
    internal class Program
    {
        public static bool Active { get; private set; } = true;
        #pragma warning disable CS8618, CS8622
        static NotifyIcon trayIcon;

        static void Main(string[] args)
        {
            trayIcon = new NotifyIcon()
            {
                Icon = SystemIcons.WinLogo,
                Text = "pauseondisconnect",
                Visible = true
            };

            trayIcon.Click += ToggleApp;

            MMDeviceEnumerator enumerator = new();
            enumerator.RegisterEndpointNotificationCallback(new AudioNotifier());

            Application.Run();
        }

        private static void ToggleApp(object sender, EventArgs e)
        {
            Console.WriteLine("hello");
            Active = !Active;

            trayIcon.Icon = Active ? SystemIcons.WinLogo : SystemIcons.Warning;
        }
    }

    internal class AudioNotifier : IMMNotificationClient
    {
        [DllImport("user32.dll")]
        static extern void keybd_event(byte bVk, byte bScan, uint dwFlags, UIntPtr dwExtraInfo);
        public const int KEYEVENTF_KEYUP = 0x0002;
        public const byte VK_MEDIA_PLAY_PAUSE = 0xB3;

        public void OnDeviceStateChanged(string deviceId, DeviceState newState)
        {
            Console.WriteLine(deviceId, newState);
        }

        public void OnDeviceAdded(string pwstrDeviceId) { }
        public void OnDeviceRemoved(string deviceId) { }
        public void OnDefaultDeviceChanged(DataFlow flow, Role role, string defaultDeviceId) {
            if (!Program.Active) return;
            if (flow == DataFlow.Render && role == Role.Console) // I'll just do like this and hope it works.
            {
                Console.WriteLine("flow: {0}, role: {1}, id: {2}", flow, role, defaultDeviceId);

                // i tried using GSMTC but fuck winrt and all that bullshit
                keybd_event(VK_MEDIA_PLAY_PAUSE, 0, 0, UIntPtr.Zero);
                keybd_event(VK_MEDIA_PLAY_PAUSE, 0, KEYEVENTF_KEYUP, UIntPtr.Zero);
            }
        }
        public void OnPropertyValueChanged(string pwstrDeviceId, PropertyKey key) { }
    }
}