Corner frequency comes from Bode Plots, you can ignore the equations and just look at

the graphs. Also, to tell what your ckt is doing, you should plot out the Bode Plot for your

amp, ie measure Gain vs Frequency.

http://en.wikipedia.org/wiki/Bode_plot

The one formula you have is useful - ONLY because the low-pass and high-pass frequencies

are 100X apart, so you can use the formula for both cases [ie, each cap separately] with

little effect from the other cap.

corner frequency (f=1/(2*pi*R*C)

For low-pass, R = R8 || (R5+R6) = 5K, and C = 10nF.

For high-pass, R = R8+R5+R6 = 20K, and C = 1uF,

so Flo = 8 hz, and Fhi = 3185 hz.

Your gain in the "mid-band" = R6 / (R8+R5+R6) = 0.1, so you're losing most of your

signal right there.

AFAICT, the purpose of R7 & C15 is to prevent high-frequency oscillations in the LM386

at high-gains, as the corner frequency for that network is very high = 318 Khz.