Here is a nice 8 fader audio mixer for your PC or MAC. It functions much as a production studio mixer would for play back.
Each channel can be loaded from your library of music and played. this would be ideal for mixing music or adding SFX.
I've played around with it and it's easy to use. If you need more than 8 faders the program can be opened a second time.
This is a ZIP FILE and is FREE. (save the file and extract after download).
(Audiomixer Player 2.1) http://langomas.com/audiomixer_win.html
Add this FREE VU meter to your desktop and You'll be able to monitor levels as you play back your material. (the VU meter has been posted elsewhere on this site along with a audio tone generator).
(VU Meter v1.1) http://www.vuplayer.com/other.php