Why are resistors necessary in a summing audio circuit?
simulate this circuit &ndash; Schematic created using CircuitLab
The resistors set the voltage ratio for the two signals. With equal sized resistors, the signals are mixed equally,
The resistors also present an electrical load to the source that is reasonably predictable.
Without the resistors, the two sources would be fighting each other at any time they disagreed about the signal voltage, leading to distortion or even smoke.