Since "ils" comes before a word starting with a vowel, I.e. "écrivent," the "s" in "ils" is not silent and can be heard if you listen closely. Otherwise it's like others have said, and you have to rely on the context of the sentence.
For anyone else having problem distinguishing the audio, I just realized that you need to play both speeds. The slow speed seems to be off in many cases, unfortunately up until just now that is the one I had been relying on...
That can be quite annoying, but I think that the way it works is that if you for example miss out a letter which changes the word into a new word, then it marks you wrong. Very annoying :(. But that's why you have to thoroughly check your answers before pressing enter. Bonne chance!