Use ffmpeg instead of audioconcat and mp3val

• Use only ffmpeg and the ffmpeg-binaries package, and remove audioconcat and mp3val.

• For joining files: ffmpeg concat demuxer (rather than regular ffmpeg concat)

• If the joined audio file is too long, then extract the correct section that represents the broadcast's time, and save this as the temporary file.

  • Example: https://stackoverflow.com/questions/7945747/how-can-you-only-extract-30-seconds-of-audio-using-ffmpeg