It's been a good number of years since I did Dispatcher Assessment but IIRC there was only one assessment and if successful then interviews straight afterwards.
You will be given one sequence of noises (i think it was 3 so e.g. Horn, Bell, Whistle) which denotes that a train is ready to start. The noises in any other order means the train can not start.
The test will then start and you will hear a large number of noise sequences with the question being "Is the train safe to start?"
If you hear "Horn, Bell, Whistle" then tick yes, any other order e.g. "Bell, Whistle, Horn" then tick no
At the same time, you will hear members of the public asking you questions about the train service. You will be asked questions as to what they were asking. Some will be nice and clear, others you will hardly hear and struggle to make out what they are actually saying.
Obviously you need to concentrate on the noise sequence, this is the most important aspect, it doesn't really matter if you heard where they wanted to go, if you can answer its a bonus but concentrate on whether the train is safe to start!
Hope that all makes sense and helps, as I say its been a while since I was Dispatching.