Buffer stage for audio filter
It is a differential non inverting amplifier with a gain of ~1, (all 4 legs have the same resistance if you ignore R18)

R18 is acting as a 6V bias to keep the output and input between the 0 to 12V supply rails of the op amp, this mainly seems like they did not want to use symmetrical supplies for the op amp, 

So the output voltage of the op amp is roughly signal in +6V

C3 is the input coupling capacitor, which removes any bias outside, and C4/R15 are to remove the DC offset this buffer has,