I recently needed to extract the important portion of one mp3 file. I also recently had the opposite problem of joining two related mp3 files. Here is how these operations can be achieved. Firstly, to extract an important portion of a given mp3 file you can use the following command:
ffmpeg -i input.mp3 -ss "0:35:26" -t 724 output.mp3
This starts the extraction at 35 minutes 26 seconds into the mp3 file called input.mp3. The extraction then runs for a duration of 724 seconds, and the output is stored in output.mp3.
To combine two mp3 files, you can simply concatenate the individual mp3 files:
cat file1.mp3 file2.mp3 > file3.mp3