Feel free to contact me here in a reply post for sound editing assistance.
If you have iMovie, you can not only edit media easily, you can edit audio, too. It comes equipped with easy access to your music library and tons of sound effects. You'll be creating sound like a professional in no time.
If you feel that iMovie is too difficult to figure out on your own, I can help you with it. If you have large edits, we can also work with my Final Cut Pro app. In a reply post, upload your mp3 music, additional mp3 audio sources, and a description of what you'd like to create. We can meet and work together, so you learn the process and achieve exactly what you'd like.