[foobiekr]

If you're going to use a less compatible format than mp3, it is far batter to just swap the container and not transcode it:

-i in.aax -c copy out.m4b

1. does not transcode 2. captures and retains all of the metadata

[lozf]

Yes, I'm usually strongly opposed to lossy transcodes, but for portable listening of speech heavy content on a space-limited device, that plays Opus easily, it can be a worthwhile trade-off for many.