Nate,
The schematic is at the end of the assembly manual (did you put your Macchiato together without the manual?).
The noise of the audio amp can be dependent on the impedance of what it’s driving. It’s designed to drive low impedance loads, like speakers, not high impedance loads like a guitar amp or something, although it does fine in that capacity in most applications. So that might be causing some of the noise. From my experience, I’ve had to be mindful of the gain structure of the Macchiato and what it’s plugged into. Try keeping the volume very low on the Macchiato and boosting the input on what it’s plugged into…or vice versa if that doesn’t work. It all depends on how the impedance of the 2 devices match. But i’ve had good luck getting the noise floor low enough for recording by playing around with these levels.
You asked about getting more of a line level rather than headphone level. There’s not much difference. Headphone level can just supply more current, but can usually get to around the same voltage level into higher impedances. But if you want, you can replace the the amp IC with a buffer (to reduce the noise caused by the amp IC). Disconnect pins 3 and 5 from the amp IC and hook in a unity gain buffer (either an opamp or emitter follower or something). You may want to disconnect R20 and R22 also if you don’t want those filters tainting the signal. You’ll want to remove the speaker too, so that the low impedance doesn’t strain the buffer.
-Brach