[CM] mixing in snd

[Bill Schottstaedt]

> Does anyone know of a way to mix files with nice movable mix tags 
> without generating huge temp files?

I've toyed with making a mix a virtual edit; you'd have only the
mixed-in sound on disk, and begin times or whatever in core,
and the edit reader would be smart about reading the mixed in
stuff at the right time -- hmm, now as I type, I think this might
not be so hard...