Check that both sides of your speakers or headphones work and are wired the right way round, then sweep through low to high frequencies to hear what your setup can reproduce.
Left / Right channel test
Leftclick to play
Bothclick to play
Rightclick to play
Click a box to play a tone through that side. You should hear Left only from the left speaker/earphone, and Right only from the right.
Start with the volume low - a sustained tone can be loud and tiring, especially in headphones.
Frequency sweep
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Plays a smooth tone rising from 20 Hz to 20 kHz over 15 seconds. Most people can't hear below ~30 Hz or above ~16 kHz, and many speakers can't reproduce the extremes either.