/diy/ - Do It Yourself

I want to build my own stereo 4-channel audio mixer as an easy novice project.
I was going to use pic related plan but somebody said that I should put an op amp on each circuit so one circuit doesn't change the tone of the others. Does this really matter though if I'm just mixing volumes?
The finished project will actually be used to mix audio in my entertainment centre so I don't want it to sound like shit.

Thanks for any help.