What sort of conversion would you need to take a output from headphones (Stereo) to a Microphone input(mono)?

I do not need to combine left and right audio (it would be nice), but I know the speaker level signal differs from a microphone input. For things to work properly it must be a microphone input level.

Incoming search terms:

• stereo outputs to mono input
• stereo headphone output conversion to line level mono

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