Randofo on Instructables shared this guide to making a simple two channel audio mixer:

This simple passive DIY stereo audio mixer demonstrates resistors in use. When I say stereo, I am not talking about your home entertainment signal, but an audio track with a separate left and right channel.

This mixer will allow us to combine two stereo tracks into one single track, while adjusting the volume of both tracks individually and together.

We will also go over techniques for mounting electronics in an enclosure. Since this is primarily a lesson in electronics and not building enclosures, it is not necessary that you mount your electronics in exactly the same way I do. However, I recommend trying to follow along.

Check out the full guide here!!