Don’t worry about it, just start listening to Listening Exercise 1 a few times and then move on. You’ll soon catch up.
The listening exercises do use up to date vocabulary (i.e. Ex 1 covers Challenges 1-5, Ex 2 Challenges 6-10 and so on), but other than that you’re not missing anything by having done them slightly late.
That’s because you probably won’t understand much of what’s going on anyway – and that’s by design. They are read at a very quick pace (by the end double speed or more by the end) in order to train you to listen to natural Welsh – you won’t get anything but if you persist, then you’ll understand more and more, and more importantly, ‘normal’ Welsh will start to sound a lot slower and therefore more understandable…
At first they will seem impossibly fast, but when you go back to Ex 1 at the end of the course, you’ll be surprised how (relatively) slow and understandable it seems.