Could you reword "isolate the DC level of the input signal"? The signal coming in would not have any DC offset since it's intended for a speaker/driver load, right?The upper .1uF cap is there to isolate the DC level of the input signal.
The lower .1uF cap keeps the junction of R3/R4 quiet. Current flowing through resistors causes electrical noise; caps to ground filters out that noise.
The only type of speaker this would drive would be a piezoelectric speaker (or maybe earbuds). Another amplifier is needed to drive coil speakers.R3 and R4 create a DC level on the + input. The amplifier is going to duplicate that DC on its - input. The top, left, 0.1 stops that DC from escaping into whatever is bringing the AC signal, and the 10 uf on the right stops that DC from running through the speaker.