Convert 3gp files to Ogg Vorbis?

Question

I have a Xamarin.Android project that records audio to the .3gp format using the MediaRecorder. I need to convert this file to the .ogg Ogg Vorbis format.

Is there a library or NuGet package that does this already? If not, what would be the best approach for writing a converter (note: I've never written an audio converter in my life)?

Thanks in advance!

EDIT: implementation needs to be cross-platform.