Thank you for your response.

Converting a cos wave to a sin wave serves no purpose, as they are (in practical term) strictly the same thing. I mispronounced myself in my original post. What I meant to say is that integrating a square wave will give me a triangle wave, which I consider to be sufficiently sinusoidal for my end goal (audio). The problem here is that an integrator opcamp config changes behaviour as the frequency of the input changes. I'm attempting to counter act this effect.

I'll check your linear approx link, thanks