Airfoil can capture from System Audio (all output), a specific app, or an audio input device. Select the source from the dropdown at the top of the main window. Only one source is active at a time.

Each AirPlay device on your network appears as a speaker tile. Click the power icon to connect. Volume is set per-speaker. Latency can be adjusted per-speaker in preferences to keep everything in sync.

Install Airfoil Satellite on other Macs or iOS devices to turn them into receivers. Free companion app, available separately.