>Or, do I just ABABAB the same track, but each time I make a decision the person in charge flips a coin that decides which source is A and which is B and I start again (repeat 5 times)?

Pretty much this. The idea is that you do the same comparison (e.g. the Depeche Mode track) multiple times and each time choose which one you think is which, with it being randomized after each decision. The more trials you do, the more you reduce out the possibility that you just happened to pick the right one by chance.