I don't know why they wrote it themselves, but if they don't want to use third party code, implementing OCB is faaaaaaaar simpler than implementing GCM or just

I don't know why they wrote it themselves, but if they don't want to use third party code, implementing OCB is faaaaaaaar simpler than implementing GCM or just about any other AEAD. But I suspect ChaCha20 would be a better fit since a stream cipher is a good fit for streaming audio.