OBS Output Tab Recording Settings

These radio buttons control which audio track is recorded. In the Output tab with Advanced output mode on, type Standard, you get checkboxes that allow recording of separate audio tracks within a single container. With Advanced output mode on and Custom type output selected, you get radio boxes where you choose which audio track you want recorded. It's really just a question of whether you want unmuxed or muxed multi track audio. If you're getting audio in the stream but NOT in the recording, try changing which audio track(s) are selected in the Output/Recording tab and see if it helps.