Can you please share a link to one of lessons you mentioned that problem with? Thanks!
I noticed that it does glitch sometimes, but very rarely.
Refreshing the page fixes it for me.
Perhaps try another browser.
I use either Firefox or Chrome, and if something does not function well on one I then find it does work well on the other. 😁
You don't mention what course you're having trouble with. There are controls to slow down the speed of the audio though (and speed up).