tool to add sfx to mp3s

what is the best tool to add sfx to mp3? eg. if i want to add gunfire to a mp3.

thanks.

I use Audacity for any sound editing I need to do.
It's user-friendly and open-source, so it's free.

Yeah, Audacity seems like a nice freeware application for simple audio editing like that. =)

i already got audacity please tell me how to do it. so far i can cut, join mp3s but i can't figure out how to add sfx to existing mp3s.

thanks

Import both sounds you want into your project, then move the special effect to wherever in the track you want to have it using the double-sided arrow tool.