skip the in rack mixer for now to reduce complexity (I'm assuming that you can get audio out of this)
go from vco to attenuator (you do have attenuators, don't you?) and from there via a 3.5mm -> 6.5mm cable into one of the channels on the mixer? can you now hear audio on either speakers connected directly to the mixer or via headphones?
I'm also assuming you can get other audio out of the mixer when you plug other sound sources into it...
if assumptions are wrong - let us know!
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities