Practice them until you master them. Press the button for the pronunciation you hear. They distinguish between a vowel /a/ and a vowel /ɤ/ that articulate it as the o in Spanish /o̞/ but without lip rounding. The button to identify what you hear has the IPA characters, and the label below it the corresponding Pinyin in red. In the image you can see the difference, and the blue line shows you the frequency variation for the tone.