Hey everyone, author of the library here.

There's a live JS demo of this you can try for yourself at https://quiet.github.io/quiet-js which demonstrates the audible and ultrasonic modes.

The JS version is compatible with the Android version. There's also iOS ( https://github.com/quiet/QuietModemKit ) and C ( https://github.com/quiet/quiet ) which interop as well.

The throughput you can get changes depending on conditions. Across a cable, you can achieve about 64kbps. Across a short air gap, about 3-4 kbps. Across a room, you can maintain 5 bps or so. There's no capacity negotiation/detection, instead you preconfigure using a modem profile.

A good demonstration of Quiet's flexibility can be seen here https://quiet.github.io/quiet-profile-lab