public static Uri DecodeAudioUrl(this Uri audioUrl)
{
    var segments = audioUrl.Segments.ToList();

    segments.RemoveAt((segments.Count - 1) / 2);
    segments.RemoveAt(segments.Count - 1);

    segments[segments.Count - 1] = segments[segments.Count - 1].Replace("/", ".mp3");

    return new Uri($"{audioUrl.Scheme}://{audioUrl.Host}{string.Join("", segments)}{audioUrl.Query}");
}
//...
audio.Url.DecodeAudioUrl();