I've found a helpful work-around for the mix button in the meantime --- in COS, go to the EQ section and then (if on Windows), hold Ctrl and select only the EQ points you want to move. Drag those up or down incrementally until you get the right blend you're looking for. :-)
You're adjusting the EQ manually...you'll notice how the spectrum line (your sound source) moves along with the EQ movements. The future "Mix" function will allow you to blend the raw unaffected signal in with the conformed/EQ'd signal.