The Y adaptor, with the 1/8" plug into the computer output, is the little 1/8" plug a TRS type (two insulating bands between barrel and tip or one insulating band there?).

With a Mackie board, which by design splits the difference between Consumer and Pro levels, may not be driven hard enough into a Line input on the mixer. There are ways to deal with that, but first need to know the above and also more about your computer and soundcard. Laptop or desktop? The little 1/8" output you are using, is it the earphone output of a laptop or the Line Output of a desktop? What Operating System? Type of PC, brand and model?


--Mac