You need FFMPEG for this one.

Get a list of multiple audio files in the directory:

ls *.mp3 | tr '\n' '|' | less

Remove the files you don't want to combine. Check that the list is accurate. Then use the resulting string as an argument for the following command, like so:

ffmpeg -i "concat:01 audio file.mp3|02 audio file.mp3|03 audio file.mp3" -acodec copy final-combined-file.mp3

For more information and more complex use cases.