From the source
Replit added system audio integration as an opt-in feature for repls.
Users enable it by creating a secret called VNC_ENABLE_EXPERIMENTAL_AUDIO with a value of 1 and restarting the repl.
The feature uses a custom RFB proxy written in Rust to transcode PulseAudio into an Opus stream in a WebM container, with an MP3-in-MPEG fallback for browsers that do not support Opus/WebM.
