The design around the opamp is all wrong.
I guess I dont understand the links?? (maybe just missing the connection?)
(I said audio sampling?)..
BTW, I think C18 should be 0.01uF [10 nF] and not 100nF, which gives a low-pass frequency at approx318 hz. C18 = 10nF will move it up to 3180 hz.
without causing a fight on the boards here.. I see the image/schematic posted really hasnt been commented on.. on how to apply (or change) for a fix? better filtering..etc..I hear its 'all wrong'.. but how? is it really even wrong??