Pre-amp connection problem
C5 is the bypass cap. You might google this term, there are many good application and lecture notes out there.
Your power supply can not response immediately to any change in the current consumption of the circuit, and therefore there will be a drop in the supply voltage during current transisents, due to the output impedance of your batteries, or due to the delay in the regulation loop of the regulator. A bypass cap is a big charge reserve, which will supply the current your circuit needs until the battery/dc supply will react. Such a cap works well if there is no -- very small -- impedance between the cap and the circuit, so place the cap very close to the supply terminal of your circuit.