Yeah, Audio Out is a really weird one... It takes the signal path after Oscs>VCF>VCA and digitizes it (at some sub sampling less than audio rate), which can then modulate other parameters... but only certain parameters... hit or miss.
By routing it to Osc Frequency you can get some weird cross modulation back to frequency, though very crude, and the source always uses the entire audio chain, so it feeds back on itself.
If you route it to one of the delay fx as destination, and modulate the time of the delay with it, you can get some really weird sounding "rubber band" type of effects... honestly though, Audio Out has real limited usage, unless you're going for more atonal experimental sounds.
The Filter Audio Mod Destination is useful though... that provides a Osc1 > Filter Cutoff Frequency Modulation. If you crank up the resonance on the filter, you'll hear it more evidently... I've found it useful for modeling transients of brass sounds by applying an envelope to Audio Mod Destination.