Neither cat nor mp3wrap are working for me. They merge the files, but the when playing the resulting file, the audio length information is wrong. That is, it show the length of the 1st part (the part which was the 1st file), then the length of the 2nd ... . And so on. Other alternative that might work? In the past I was happy with ffmpeg but it was deprecated. 
Merge mp3 files
mp3