NAudio basic example of how to begin a fade out after a certain number of milliseconds have elapsed

DelayFadeOutSampleProvider.cs

// Define other methods and classes here
/// <summary>
/// Sample Provider to allow fading in and out
/// </summary>
public class DelayFadeOutSampleProvider : ISampleProvider
{
    enum FadeState
    {
        Silence,
        FadingIn,
        FullVolume,
        FadingOut,
    }
    
    private readonly object lockObject = new object();
    private readonly ISampleProvider source;
    private int fadeSamplePosition;
    private int fadeSampleCount;
    private int fadeOutDelaySamples;
    private int fadeOutDelayPosition;
    private FadeState fadeState;
    
    /// <summary>
    /// Creates a new FadeInOutSampleProvider
    /// </summary>
    /// <param name="source">The source stream with the audio to be faded in or out</param>
    /// <param name="initiallySilent">If true, we start faded out</param>
    public DelayFadeOutSampleProvider(ISampleProvider source, bool initiallySilent = false)
    {
        this.source = source;
        this.fadeState = initiallySilent ? FadeState.Silence : FadeState.FullVolume;
    }
    
    /// <summary>
    /// Requests that a fade-in begins (will start on the next call to Read)
    /// </summary>
    /// <param name="fadeDurationInMilliseconds">Duration of fade in milliseconds</param>
    public void BeginFadeIn(double fadeDurationInMilliseconds)
    {
        lock (lockObject)
        {
            fadeSamplePosition = 0;
            fadeSampleCount = (int)((fadeDurationInMilliseconds * source.WaveFormat.SampleRate) / 1000);
            fadeState = FadeState.FadingIn;
        }
    }

    /// <summary>
    /// Requests that a fade-out begins (will start on the next call to Read)
    /// </summary>
    /// <param name="fadeDurationInMilliseconds">Duration of fade in milliseconds</param>
    public void BeginFadeOut(double fadeAfterMilliseconds, double fadeDurationInMilliseconds)
    {
        lock (lockObject)
        {
            fadeSamplePosition = 0;
            fadeSampleCount = (int)((fadeDurationInMilliseconds * source.WaveFormat.SampleRate) / 1000);
            fadeOutDelaySamples = (int)((fadeAfterMilliseconds * source.WaveFormat.SampleRate) / 1000);
            fadeOutDelayPosition = 0;
            
            //fadeState = FadeState.FadingOut;
        }
    }

    /// <summary>
    /// Reads samples from this sample provider
    /// </summary>
    /// <param name="buffer">Buffer to read into</param>
    /// <param name="offset">Offset within buffer to write to</param>
    /// <param name="count">Number of samples desired</param>
    /// <returns>Number of samples read</returns>
    public int Read(float[] buffer, int offset, int count)
    {
        int sourceSamplesRead = source.Read(buffer, offset, count);
        
        lock (lockObject)
        {
            if (fadeOutDelaySamples > 0)
            {
                int oldFadeOutDelayPos = fadeOutDelayPosition;
                fadeOutDelayPosition += sourceSamplesRead / WaveFormat.Channels;
                if (fadeOutDelayPosition > fadeOutDelaySamples)
                {
                    int normalSamples = (fadeOutDelaySamples - oldFadeOutDelayPos) * WaveFormat.Channels;
                    int fadeOutSamples = (fadeOutDelayPosition - fadeOutDelaySamples) * WaveFormat.Channels;
                    // apply the fade-out only to the samples after fadeOutDelayPosition
                    FadeOut(buffer, offset + normalSamples, fadeOutSamples);

                    fadeOutDelaySamples = 0;
                    fadeState = FadeState.FadingOut;
                    return sourceSamplesRead;
                }
            }
            if (fadeState == FadeState.FadingIn)
            {
                FadeIn(buffer, offset, sourceSamplesRead);
            }
            else if (fadeState == FadeState.FadingOut)
            {
                FadeOut(buffer, offset, sourceSamplesRead);
            }
            else if (fadeState == FadeState.Silence)
            {
                ClearBuffer(buffer, offset, count);
            }
        }
        return sourceSamplesRead;
    }

    private static void ClearBuffer(float[] buffer, int offset, int count)
    {
        for (int n = 0; n < count; n++)
        {
            buffer[n + offset] = 0;
        }
    }

    private void FadeOut(float[] buffer, int offset, int sourceSamplesRead)
    {
        int sample = 0;
        while (sample < sourceSamplesRead)
        {
            float multiplier = 1.0f - (fadeSamplePosition / (float)fadeSampleCount);
            for (int ch = 0; ch < source.WaveFormat.Channels; ch++)
            {
                buffer[offset + sample++] *= multiplier;
            }
            fadeSamplePosition++;
            if (fadeSamplePosition > fadeSampleCount)
            {
                fadeState = FadeState.Silence;
                // clear out the end
                ClearBuffer(buffer, sample + offset, sourceSamplesRead - sample);
                break;
            }
        }
    }

    private void FadeIn(float[] buffer, int offset, int sourceSamplesRead)
    {
        int sample = 0;
        while (sample < sourceSamplesRead)
        {
            float multiplier = (fadeSamplePosition / (float)fadeSampleCount);
            for (int ch = 0; ch < source.WaveFormat.Channels; ch++)
            {
                buffer[offset + sample++] *= multiplier;
            }
            fadeSamplePosition++;
            if (fadeSamplePosition > fadeSampleCount)
            {
                fadeState = FadeState.FullVolume;
                // no need to multiply any more
                break;
            }
        }
    }

    /// <summary>
    /// WaveFormat of this SampleProvider
    /// </summary>
    public WaveFormat WaveFormat
    {
        get { return source.WaveFormat; }
    }
}

Program.cs

void Main()
{
    using(var reader = new AudioFileReader(@"D:\Audio\Music\Example.mp3"))
    {
        var fadeOut = new DelayFadeOutSampleProvider(reader);
        fadeOut.BeginFadeOut(10000, 2000);
        using(var player = new WaveOutEvent())
        {
            player.Init(fadeOut);
            player.Play();
            while(player.PlaybackState == PlaybackState.Playing)
            {
                Thread.Sleep(500);
            }
        }
    }
}