Using diodes to mix 2 audio signals into one set of speakers
As mentioned in the comments, diodes are not the way to go. They will massively corrupt your audio signal, and it will just sound utter rubbish.

The simplest method is to use a passive mixer.  That simply consists of two resistors per channel:







simulate this circuit &ndash; Schematic created using CircuitLab

You can tie all the ground wires together without resistors.

This of course is assuming you are using a powered speaker set, and not just raw speakers.