When accessing a meeting online using a web app through macOS, the only audio output that will be available is the inbuilt system default microphone.
As highlighted below
If you wish to use an alternative audio output you can switch this through the system preferences of your Mac. As follows -
On your Mac, choose Apple menu > System Preferences, click Sound, then click Output.
Select the device you want to use in the list of sound output devices.
Once the new device has been selected internally this will then be the preferred audio output device when you rejoin your meeting.
For more details you can visit the Apple macOS User Guide
If you continue to have any issues with your audio please contact support who will be able to assist you further.
T: +44 (0) 203 137 8933