Convert a folder one m4b at a time to opus. Edit line 4 for more or less any file format that contains audio.

gistfile1.txt

#!/bin/bash

# Loop over all m4b files in the current directory
for input in *.m4b
do
    # Print a message indicating the file being processed
    echo "Processing file $input..."

    # Use ffmpeg to convert the m4b file to a wav file
    ffmpeg -i "$input" "${input%.*}.wav"

    # Use opusenc to convert the wav file to an opus file
    opusenc --downmix-mono --bitrate 32 --vbr "${input%.*}.wav" "${input%.*}.opus"

    # Remove the wav file
    rm "${input%.*}.wav"
done