Hi, JohnL123. what happens if you hold down the "Option" key and click on the speaker icon at the top right of your screen ? The speakers should be listed under USB Output....If you go to system preferences, then sound, then sound effects/output, it should be there, in theory....You might also have to mess around with Audio MIDI setup and ensure that your sound output is set to the B&W MM1 speakers. You should make sure the sound is set to the default, at least for now (44 Khz, 16 bit 2 channel.) ....you can play a test tone to ensure that your speakers are working fine. I don't know what MacOS you have/are running, if that would make a difference.... maybe they need a firmware update ?? you could try doing a reset smc/pmu and zap pram 4 times on startup on your iMac. here's the link to ask Bowers and Wilkins a question. I'm sure they could help you out better than I could: https://support.bowerswilkins.com/app/products/detail/p/6369/locale/en-ca
john B