ffmpeg from 5.1 or 7.1 audio in separate mono streams to stereo

Question

I have a file with 7.1 audio, where each channel is in a separate mono stream. How can I convert all these mono streams to plain stereo?

(The documentation suggests to use the -ac 2 option, but that is meant for converting 5.1 in one stream of 6 channels, not 8 separate streams)

The audio I have has the streams labelled, presumably correctly, so that might help:

$ mediainfo "$in" | egrep '^(Audio|Channel)'
Audio #1
Channel(s)                               : 1 channel
Channel layout                           : Lt
Audio #2
Channel(s)                               : 1 channel
Channel layout                           : Rt
Audio #3
Channel(s)                               : 1 channel
Channel layout                           : L
Audio #4
Channel(s)                               : 1 channel
Channel layout                           : R
Audio #5
Channel(s)                               : 1 channel
Channel layout                           : C
Audio #6
Channel(s)                               : 1 channel
Channel layout                           : LFE
Audio #7
Channel(s)                               : 1 channel
Channel layout                           : Ls
Audio #8
Channel(s)                               : 1 channel
Channel layout                           : Rs

Update: In fact, the file I have is not 7.1, but Stereo + 5.1. So I already have the stereo tracks that I want, correctly labelled "Lt" and "Rt".

But there is still an aspect in the question which I would like to learn: how to convert 7.1 or 5.1 separate mono streams to a single stereo stream?

Answer 1

-ac 2 will work for any number of input channels. The example is just one use-case.