Convert Many mp3's into one track?

Hey guys/gals, a client wants me to convert a few of their song which are in mp3 format or WMA format into on big track for people to download off their site. Their is about 15 tracks to make into one track. I dont work with sound much so i dont know how i would do this and what program to use.

Thanks in Advance

i'd say audacity (free/open source) should do the job. there you can open all the soundfiles and copy/paste them together