Similar to images and video, in order to evaluate the audio quality you’ll need to find either unedited video or separate sound files. First recommendation is to put your headphones or earphones on while playing back the video. That’s the best way to hear if the sound is muffled or too low. Try to find samples which show someone playing a music instrument or simply a person talking into a camera.
You should also be aware that most low-budget cameras do not have a perfect sound. In most cases they are not good for extremely loud music or concerts, as their microphones are just not that high-quality.
If the audio quality is really important (for instance you are a guitarist, pianist, vlogger, or just an audio-freak), I recommend that you look for a camera which supports an external microphone. Those kind of cameras are in mid and high-end range, but you get what you pay for. External mic is a must if you need good sound and want to use an action camera while doing any sort of physical activity.
One additional possibility (especially if the camera you are interested in, does not support external mic) is that you use an external voice recorder and merge the video with the recorded audio in video-editing software. Since iPhone has decent enough mic, you can also record the sound with a smartphone. However these ways of recording a sound are too robust for being used in action or in an extreme situation, they are on the other hand quite OK for concert and celebration recording.