[BUG] Opus output audio cannot be mono?

[Selur]

Looking at the debug output and the encoding call:

ffmpeg -y -threads 8 -loglevel fatal -nostdin -i "C:\Users\djoens\AppData\Local\Temp\iId_3_aid_0_lang_en_2021-07-30@09_46_42_1810_01.aac" -ac 2 -ar 44100 -f sox - | sox --multi-threaded  --temp "C:\Users\djoens\AppData\Local\Temp\2021-07-30@09_46_42_181001" --buffer 524288 -S -t sox - -t flac - | opusenc --bitrate 12 --comp 10 --framesize 20 --expect-loss 0 --max-delay 1000 --ignorelength -  "C:\Users\djoens\AppData\Local\Temp\iId_3_aid_0_lang_en_2021-07-30@09_46_42_1810_02.opus"

the downmix is missing in the 'sox'-part of the call.
-> I can reproduce the issue and will look into it after work today.

Cu Selur