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If you want to convert mp3 files to low-bandwidth Opus, you can do it in a command line environment using ffmpeg:

ffmpeg -i input.mp3 -c:a libopus -b:a 16k -ac 1 output.opus

-i input.mp3

input file

-c:a libopus

output opus format

-b:a 16

use 16kbps bandwidth

-ac 1

convert to 1 channel

output.opus

output file