Decode AMR in C#: .amr, .awb and 3GP to WAV

dotnet new console -o DecodeAmr && cd DecodeAmr
dotnet add package RecursiveAI.OneAudio --version 1.4.0
t=https://raw.githubusercontent.com/recursive-ai-oss/oneaudio-csharp/v1.4.0/testdata
curl -L -o memo.amr $t/containers/synthetic-nb.amr
curl -L -o memo.awb $t/containers/synthetic-wb.awb
curl -L -o memo.3gp $t/containers/amrnb.3gp

using OneAudio;
using OneAudio.Amr;
 
// A kit that checks for MP4 and AMR before MP3 (see "If Audio.Open fails").
var kit = new AudioKit(new AudioCodecRegistry()
    .AddWav().AddMp4().AddAmr().AddAac().AddMp3());
 
foreach (string path in new[] { "memo.amr", "memo.awb", "memo.3gp" })
{
    using IAudioDecoder decoder = kit.Open(path);
    AudioFormat f = decoder.Format;
    Console.WriteLine($"{path}: {f.SampleRate} Hz, {f.Channels} ch, " +
                      $"{decoder.LengthFrames / (double)f.SampleRate:F2} s");
}
 
// AMR to WAV.
TranscodeResult wav = kit.Transcode("memo.amr", "memo.wav");
Console.WriteLine($"memo.wav: {wav.Frames} frames, " +
                  $"{new FileInfo("memo.wav").Length:N0} bytes");
 
// At the rate and channels your app mixes at.
var at48k = new LoadOptions { SampleRate = 48000, Channels = 2 };
AudioClip memo = kit.Load("memo.awb", at48k);
Console.WriteLine($"memo.awb at 48 kHz stereo: {memo.Frames} frames");
 
// Frames from a network stream go straight to AmrNbDecoder: a table-of-contents
// byte plus the speech bits, 20 ms each. Here they come from memo.amr.
byte[] file = File.ReadAllBytes("memo.amr");
var nb = new AmrNbDecoder();
short[] pcm = new short[AmrNbDecoder.SamplesPerFrame];
int offset = 6, decoded = 0;                     // skip the "#!AMR\n" magic
while (offset < file.Length)
{
    int type = AmrFrameTable.FrameType(file[offset]);
    int size = 1 + AmrFrameTable.PayloadBytes(false, type);
    if (decoded == 50)
        nb.DecodeLost(pcm);                      // a lost packet, concealed
    else
        nb.Decode(file.AsSpan(offset, size), pcm);
    offset += size;
    decoded++;
}
Console.WriteLine($"AmrNbDecoder: {decoded} frames, one concealed");

memo.amr: 8000 Hz, 1 ch, 3.00 s
memo.awb: 16000 Hz, 1 ch, 3.00 s
memo.3gp: 8000 Hz, 1 ch, 3.00 s
memo.wav: 24000 frames, 48,080 bytes
memo.awb at 48 kHz stereo: 144000 frames
AmrNbDecoder: 150 frames, one concealed