How you describe good sound quality depends on its use. For an average adult, the sound should neither be too loud, nor too low. The same also goes with frequencies, as different audio devices (like speakers) produce sound differently. The frequencies should be a balance of high and low, to get a desirable, comfortable sound which does not hurt the ears.
However, different people have different preferences and uses for these devices.
For gamers, they usually opt to go with headphones, where the dynamic sound effects and low range ambience effects can be heard properly. The can use their everyday earphones, but they won't sound as good with the game, as they perform splendidly with high frequencies, but not as good with the low frequencies.
Another example would be music production, wherein the producer needs the perfect balance of frequency response and volume as well, to properly be able to hear what the music actually sounds like. This is done using studio monitors, which are special speakers, designed specifically for this task. These producers can also use basic earphones, but they won't be able to tell if the low or some mid range frequencies are correct, as the tiny speakers won't be able to produce those frequencies.
These are examples, but the thing us the same at last, the sound should be clear, comfortable and little to no interference noises or any other such artifact should be present in the final sound.
Carlos Alves de Sousa
