5.1 surround isn’t detected

The upmix only switches on when your Mac’s sound output has 6 or more channels. Work through these checks:

1. 1In System Settings › Sound › Output, make sure the HDMI receiver is selected, not the Mac speakers or headphones.
2. 2In Audio MIDI Setup, select the HDMI device and set Format to 6 ch. If only 2 channels are offered, the receiver or TV in between isn’t telling the Mac it can take 5.1. Connect the Mac directly to the receiver, or check the receiver’s HDMI audio settings.
3. 3Click Configure Speakers… and choose 5.1 Surround.
4. 4Switch back to Paatu Cassette. It checks again when its window becomes active. If the card still says stereo, quit and reopen the app.