This page describes different tools you can use to convert MP3 audio files to Ogg Vorbis format. If you know about a technique that's not listed here, please feel free to add it.
Note: If you have the recording in a lossless format, you should convert from that. MP3 and Ogg Vorbis are both lossy formats and the loss adds up when you convert from one to the other.
Sound Converter is a GNOME application that makes it easy to convert MP3 files into Ogg Vorbis format. You can use it to convert many files in one batch, and Sound Converter will save all of the metadata tags when you do.

