Set audio balance using core audio api

BalanceExtensions.cs

public static class BalanceExtensions
{
    // left=0, right=1
    private const int LeftChan = 0;
    private const int RightChan = 1;

    /// <summary>
    /// Gets the ratio of volume across the left and right speakers in a range between -1 (left) and 1 (right). The default is 0 (center).
    /// </summary>
    /// <param name="volume">The audio volume endpoint</param>
    /// <returns></returns>
    public static float GetBalance(this AudioEndpointVolume volume)
    {
        VerifyChannels(volume);

        var masterVol = Math.Max(1e-6f, volume.MasterVolumeLevelScalar);

        var leftVol = volume.Channels[LeftChan].VolumeLevelScalar;
        var rightVol = volume.Channels[RightChan].VolumeLevelScalar;

        var balance = (rightVol - leftVol)/masterVol;
        return balance;
    }

    public static void SetBalance(this AudioEndpointVolume volume, float balance)
    {
        VerifyChannels(volume);

        var safeBalance = Math.Max(-1, Math.Min(1, balance));
        var masterVol = volume.MasterVolumeLevelScalar;
        var rightVol = 1f + Math.Min(0f, safeBalance);
        var leftVol = 1f - Math.Max(0f, safeBalance);

        volume.Channels[LeftChan].VolumeLevelScalar = leftVol * masterVol;
        volume.Channels[RightChan].VolumeLevelScalar = rightVol * masterVol;
    }

    public static AudioEndpointVolume GetDefaultVolumeEndpoint()
    {
        return new MMDeviceEnumerator()
            .GetDefaultAudioEndpoint(DataFlow.Render, Role.Multimedia)
            .AudioEndpointVolume;
    }

    private static void VerifyChannels(AudioEndpointVolume volume)
    {
        if (volume == null) throw new ArgumentNullException(nameof(volume));
        if (volume.Channels.Count < 2)
            throw new InvalidOperationException("The specified audio endpoint does not expose left/right volume channels");
    }
}

Use with NAudio or roll your own wrapper for the Core Audio API

Here a simple test panning left-right:

[Test]//, Explicit]
public void BalanceTest()
{
    var volume = _device.AudioEndpointVolume;

    var duration = TimeSpan.FromSeconds(5);
    const int n = 50;

    var dt = (int)(duration.TotalMilliseconds/n);
    for (var i = 1; i<=n; i++)
    {
        var balance = (float) Math.Sin(2*i*Math.PI/n);
        volume.SetBalance(balance);

        Thread.Sleep(dt);

        var eps = 1e-6f / Math.Max(1e-6f, volume.MasterVolumeLevelScalar);
        volume.GetBalance().Should().BeApproximately(balance, eps);
    }
}