I have no sound when using my Mac Mini. I have Logitech speakers in the headphone jack and get system sounds and iTunes for example, but no sound from Zwift.
Are you using HDMI for the video for Zwift? It might be sending the sound over it. Check you sound out setting under the Mac Settings to see what it us using as Output. I say this as I use a Mac Mini and have the sound out over HDMI to play from the TV I have it hooked up to.
I did look at the sound settings and it was defaulted to headphones , but I am also connect to a monitor via hdmi . I’ll check on my TV to see if it is any different, thanks.