MenuForm.cs

using FftSharp;
using Microsoft.VisualBasic;
using NAudio.CoreAudioApi;
using NAudio.CoreAudioApi.Interfaces;
using NAudio.Wave;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using static System.Windows.Forms.VisualStyles.VisualStyleElement;

namespace AudioMonitor
{
    public partial class MenuForm : Form
    {
        public readonly MMDevice[] AudioDevices = new MMDeviceEnumerator()
            .EnumerateAudioEndPoints(DataFlow.All, DeviceState.Active)
            .ToArray();

        public MenuForm()
        {
            InitializeComponent();

            foreach (MMDevice device in AudioDevices)
            {
                string deviceType = device.DataFlow == DataFlow.Capture ? "Input" : "Output";
                string deviceLabel = $"{deviceType} - {device.FriendlyName}";
                lbDevice.Items.Add(deviceLabel);
            }

            lbDevice.SelectedIndex = 0;
        }

        private WasapiCapture GetSelectedDevice()
        {
            MMDevice selectedDevice = AudioDevices[lbDevice.SelectedIndex];
            return selectedDevice.DataFlow == DataFlow.Render
                ? new WasapiLoopbackCapture(selectedDevice)
                : new WasapiCapture(selectedDevice, true, 10);
        }

        private void button2_Click(object sender, EventArgs e)
        {
            WasapiCapture captureDevice = GetSelectedDevice();
            this.WindowState= FormWindowState.Minimized;
            new FftMonitorForm(captureDevice).ShowDialog();
            this.WindowState= FormWindowState.Normal;
        }
    }
}