Here’s my Bash script for combining a directory of .mp3 files into a single .mkv container using ffmpeg. It also reëncodes the audio using the opus codec which can help smooth out weird encoding artifacts from sloppily created MP3 files. The script retains the original MP3 file partitions as seekable chapters named after the input file names. I find it useful for some audiobooks purchased from libro.fm which are only available in MP3 format.