But you don't necessarily know they're guessing what you're testing for in the A/B test. If they are answering which one sounds better, some songs will genuinely sound better with a little lossy compression. Did you check to see if any audio samples deviated from the base 50% rate in either direction? For example if 70% of people chose the compressed version of Audio Sample 15, that still demonstrates an ability to discern a difference. It just turns out they like the lossy sample more.
For a contrived example, imagine an A/B test where you have to tell me which image has more red. Image 1 is a dark red panel on the left and a fully bright white panel on the right. Most people would say the left is more red, but in my fictional test it is actually the white panel because (100, 0, 0) has less red than (255, 255, 255).
If you use ABX, people know exactly what they are supposed to be matching.
For a contrived example, imagine an A/B test where you have to tell me which image has more red. Image 1 is a dark red panel on the left and a fully bright white panel on the right. Most people would say the left is more red, but in my fictional test it is actually the white panel because (100, 0, 0) has less red than (255, 255, 255).
If you use ABX, people know exactly what they are supposed to be matching.