Trouble shooting new Due build.

Hey guy's,
I finished my Due some time ago but I'm having issues getting the unit to pass sound through the arduino.
Lost interest for a while but I've since come back to it.
I've confirmed the voltages are good at the test points. -ve voltage off tp5 seemed a bit weak (-4.5 or so) otherwise everything was exactly as spec'd.
I've run the sine wave program to confirm the output section is working. seems to work fine. I get a sine wave and can control the frequency and volume.
I've written a small program to confirm the LED works and I get input from the 3 pots, 1 switch and that ADC's are receiving signal (by outputting the signal for each to the monitor). Everything looked correct to me...

The one odd thing I seem to be getting is the mix switch does not function as intended. when the switch is one way I get zero signal. when the switch is the other way I either get no signal or sometimes I get a really distorted but quiet signal.
Another odd happening that I'm getting is that the arduino makes a weird buzzing noise when first turned on. if I push the reset button once or twice it goes away... I'm hesitant to replace it as its an expensive piece so im looking to confirm everything I can before I decide it's the arduino board. (want to make sure it's not something in my pedal shield feeding a voltage someplace it doesn't below that's causing the arduino board to make the noise...)

I'm guessing I mixed up R15 or R16 with something. I did a visual check and all components looked correct...

Can someone please confirm how the mix switch works (which way is on and which way is no mix)
Also the schematic is not exactly 100% clear as to where the "in buffered" signal comes from that feeds into that mix switch. the entire line coming off the first op amp is labeled in buffered but it doesn't show it branching off to go to the mix switch???

Help?

***Resolved***
I had a few questionable connections on the output IC socket and one that I think I missed entirely.
I reflowed the questionable connections and soldered the last one and everything seems 100% now.