Naively mixing two (or perhaps more) audio signals
The simplest way to combine two signals is with two resistors as shown in the circuit diagram below.







simulate this circuit &ndash; Schematic created using CircuitLab

The ground connecton is shared between both inputs and output. The quality can be improved by adding an output buffer (opamp), but that complicates things in requiring an IC, power supply, (de)coupling caps, ...







simulate this circuit

The advantage of the lower circuit is that the inputs see a constant input impedance, one that doesn't change with the other signal's level. This improves output quality. Practically speaking the inverting output (-) of the amplifier is virtual ground, which means it is set at a static voltage (4.5V in this case). The series capacitors are required to decouple the DC bias of both in- and outputs, they allow the signal to pass, but block the DC voltage.