Trace the current path for the (-) end of C3 back to the ground pin of the amplifier. Tooooooo long. C3 should be from Power to ground of the amplifier. I would move it to just right of the amplifier.
C1 has a long loop back to the ground pin.
I would move the inputs to the top of the PCB. Run the traces from input connector down to the amplifier. This would allow a very large area of ground between the pins of IC1. Then move C1, C2 down so their (-) leads will be close to the ground pin of IC1.
R3, R4 should go back closer to IC1.