Audio not working on computer

I am trying to use Memrise for the Citizen Potawatomi language but am unable to get audio to work. I have a MacBook Pro using version 10.13.5 of High Sierra, and it doesn’t seem to matter whether I use Chrome, Safari or Firefox - in each case I can see a word, click on play and no volume. All computer settings are fine, and other things on my computer do play. Any assistance would be welcome.