I was trying to merge several MP3 files into a single MP3 the other day. I thought VLC should be able to do the job and I did find instructions for that. However, the instructions mentioned a "Open Media" option that I don't have on my version of VLC on OS X (even after updated to latest) I know for sure Audacity can do that. However, moving the sound data so that it goes one file after another seems to be a lot of work. Thanks to this post. I found there is actually a very easy way to do so:
1) File->Import->Audio (can multi-select)
2) Tracks->Align Tracks->Align End to End
3) Now you can export the whole thing to one MP3.
Isn't that simple?