My recommendation for podcast audio is:
- If the OPUS file is mainly intended for listening, choose 32kbit/s with VBR. This will cause a little distortion to the voices
- If the OPUS file is mainly intended for archival and later listening, choose 48 kbit/s with VBR. The difference between a WAV file and a OPUS file with 48 kbit/s for speech data is hardly distinguishable even using headphones, unless you specifically compare the data (as in A-B tests).
- For pure archival – in order to reduce generational loss – you can of course use lossless FLAC encoding. However, practically speaking, a 64kbit/s (or 96kbit/s if you’re just too paranoid) VBR-enabled OPUS file is so transparent that it’s hardly worth spending the huge amount of hard drive space using FLAC.
Note that VBR (variable bitrate) should always be enabled for speech data, since most podcast/speech-like data contains lots of silence. Therefore, using pure constant bitrate is strongly discouraged.