MediaDataStream を使う

MyMediaPlayer.cs

public class MyMediaPlayer
{
    private Android.Media.MediaPlayer MediaPlayerInstance { get; set; }
    
    public void CreateMediaPlayer()
    {
        this.MediaPlayerInstance = new Android.Media.MediaPlayer();
    }
    
    public void Play(System.IO.Stream stream)
    {
        var source = new StreamMediaDataSource(stream);
        
        this.MediaPlayerInstance.Reset();
        this.MediaPlayerInstance.SetDataSource(source);
        this.MediaPlayerInstance.Prepare();
        
        this.MediaPlayerInstance.Start();
    }
}

StreamMediaDataSource.cs

public class StreamMediaDataSource : Android.Media.MediaDataSource
{
    private System.IO.Stream MediaStream { get; }

    public override long Size => this.MediaStream.Length;

    public StreamMediaDataSource(System.IO.Stream stream)
    {
        this.MediaStream = stream;
    }

    public override void Close()
    {
        this.MediaStream.Close();
    }

    public override int ReadAt(long position, byte[] buffer, int offset, int size)
    {
        // 読み取りしたい位置とかがくるのでシークして読んで返す
        this.MediaStream.Seek(position, System.IO.SeekOrigin.Begin);
        var result = this.MediaStream.Read(buffer, offset, size);
        return result;
    }
}