sudo apt-get install audacity
After open the program and add your music select for delete stretch that you want and export the file for .mp3.
for know how is possible cut the files you can see this kb:
after that, execute that for all files thtat you want start the merge executing this procedure in terminal.
ffmpeg -i "concat:1.mp3|2.mp3|3.mp3|4.mp3|55.mp3" -acodec copy output.mp3