Maybe you need drivers? For you sound card?
Have you installed the drivers? It's usually realtek hd audio.

I can confirm - this is also happening in Chrome and Edge, so probably not browser related.

The sound works fine if you click on a sentence, or if you see it in the search results, but it doesn't work in the random sentence widget.

So to reproduce you need to click "Show another" button on the main page a few times to get a sentence with audio. For example, I happened to see this sentence when I was experimenting:

https://i.imgur.com/T4FnEim.png

Then you click on the loudspeaker button and there's no sound.

Once you click on the sentence itself to proceed to its page (#6559124 in this case) the sound works fine.

So the problem is probably with the Random Sentence widget (or however you call this feature).

EDIT: I played a bit more with it and the pattern is really weird.

So if you open the main page, and the Random sentence you see first (or one of its translations) does have a sound, it plays fine, and all the subsequent sentences with sound that you get after you press "Show another" work fine too.

If you open the main page and the first sentence you see doesn't have a sound (and neither do all the translations of this sentence), then you press "Show another" enough times to get a sentence with a sound, the sound wouldn't play.