Well if you wanted to do it through terminal, it would simply be the following command in the directory with your mp3s which you want to combine...
Code:
cat first.mp3 second.mp3 third.mp3 > combined.mp3
Now I will say that merging mp3s will most likely mess up any ID3 meta data that the mp3s have, so if you care about the meta data, you'll probably have to throw the newly created mp3 into itunes or something and edit artist, title, album, etc from there yourself.
A second option would be to use
Audacity which is a free open source sound editor.
The last thing I came across
JoinTogether which basically merges or breaks up mp3s and allows you to modify meta data for the newly combined mp3 all in one interface. I've never used this app, or know how well it works, but figured I'd toss it in, in case you wanted to try it out. It seems that some of its features are free, and some you need the paid version for, so not sure if the free version will work for you needs or not.
