AudioCast is two halves: the phone app receives, this one sends. Install it on the computer whose audio you want to hear.
Streams system audio through the AudioCast virtual audio device, installed from inside the app.
Download for macOSbrew install --cask jobtools/tap/audiocast
You're on a phone — this app goes on the computer you want to hear. Open audiocast.sskplay.com/download there.
On Linux the sender is built from source; there is no prebuilt download.