Practice them until you master them. Press the button for the pronunciation you hear. They distinguish between an /s/ and a /ts/. The button to identify what you hear has the IPA characters, and the label below it the corresponding Pinyin in red. Listen to a high-frequency fricative sound, as shown in the figure, in the case of the /s/. In the image you can see the difference, and the blue line shows you the frequency variation for the tone.