To get sound in mol, a few steps are necessary:
(i) Put the "MOLAudio" extension into the extensions folder the restart MacOS (i.e. mol). The extension is located in the "MOL" disk image which mounted automatically when mol is started (unless 'mount_mol_disk: no' is specified).
(ii) Open the Sound control pannel and select "Audio out". Make sure "MOL Audio" is the active audio-out device.
Note 1: Audio will not work unless linux supports the sound hardware. It should be possible to select the mol audio driver but there will be no sound.
Note 2: The "Monitor and Sounds" panel which replaces the Sound control panel on some systems might not support audio out selection. Usually, the Sound control panel is then present in a separate folder (named Apple Extras, or something similar). Copying the sound control panel from another system ought to work too.
Finally, it is possible to enable the start sound (one way of making sure sound works on the linux side):