Tape recorders, computers and capture cards all seem to be happy to accept a microphone input for adding audio or voice-overs, but very few have a “line input” to overlay music or any other “strong” signal source instead.
While you can -in theory- connect a sound source there (at your own risk) you will find that in most cases you will get pretty distorted audio. This is because microphones normally work at a much lower signal level than regular “line” in/out audio, and have a DC offset to enable simple electret microphones to work with them, which is what most passive headset/desk mics actually are.
If you want to properly connect something there that is not a microphone, you need to (at the very least) attenuate the signal, and hopefully get rid of the DC offset.
Audio attenuator “pads” exist for this kind of use case, and it’s not hard to build one yourself. This page compiles a few options for different attenuation settings and use cases. For my needs I picked the one with the -20dB and DC-offset removal.
The circuit looks like this:
I free-form soldered this circuit and tested it on my computer, and seemed to work fine, so decided to build a small enclosure for it and 3d-print it. As usual, I’ve uploaded the model to Thingiverse.
The small case should be able to accommodate most attenuation circuits, so you can build yourself a set of different attenuators for all your audio needs.