For the custom collections the audio depends on your browser/device at the moment. Most of the the bigger languages (Spanish, French, etc.) should be available on most browsers/devices (tends to work best on the latest Chrome and Safari).
We’re working on getting text-to-speech support added such that it’s generated and saved as sentences are added to the collection (for the languages that offer audio for the Clozemaster collections - Fast Track, Most Common Words, etc.) and will work consistently across browsers/devices. Soon!