However, I don't know the format of MP3 files. If they have headers (which is likely), then simply appending the start of one to the end of the next won't work. You'll end up with a corrupted file unified_file.mp3.
It's likely then you'll have to obtain an editor for MP3 files, open the first file, then paste in the other files one by one. I'm sure there's a freeware, shareware or Open Source version of one that you could use.