Two GMs Tools i want you to know about...first, a little older, the Mutant Sound Mixer:

http://www.stefanisberg.com/mutant/Mixer_eng.html

And for automatically, dynamically changing "Nature" Sounds, the Virtual Ambience, which i found today:

http://download.cnet.com/Virtual-Ambience/3000-2170_4-10657998.html

The second one gets special applause from me, since i has everything built in, and is ready to play a changing forest ambience out of the box.

Both a free as far as i understand. The first one can dish out most ambience sounds you need, even if it´s a little repetitive. And its customizable.

thanks! will try it out

sweet finds there my good man!