Okay it's now happening with other previous projects that I know were playing back this type of comp successfully, so I think it may be a subtle bug in 18.2. Reverting to 18.1 for now to see if that helps
Is any audio playing back? What happens if you drag a piece of working audio into a brand new comp? If that's not playing back then I would check your audio output settings in the preferences. I would also check the Preview panel to make sure the speaker icon is selected. When doing this, make sure the way you're choosing to playback is also selected. For example, if you're starting your playback with the Spacebar then make sure the Preview settings for Spacebar are selected, vs, say, the "0" key on the numPad.