Using mpv as a white noise mixer

[avidseeker]

My goal basically is to mimic the functionality of these websites: https://mynoise.net/ https://asoftmurmur.com/

Say you have rain.flac, waves.flac, fire.flac, how can I use mpv to play them all simultaneously in a single daeomnized instance? Bonus: how can I adjust the volumes of each to have for example rain.flac playing at 70% volume, waves.flac at 50% volume, and so on?

[avidseeker]

Found my answer in --external-files and --lavfi-complex options.