Star sounds in Sky Guide are based on two things: temperature and luminosity. These two properties help to classify stars and tell us how far they have undergone stellar evolution.
In Sky Guide, stars with higher surface temperatures have higher pitched notes. For example, a very hot star like Alnitak will have a high note and a relatively cool star like Betelgeuse will have a low note. Hot stars are massive objects that live fast and die young. See if you can find them by listening to their notes!
Additionally, stars with greater luminosity will sound louder. You might notice that stars which are more luminous (louder) don't necessarily appear brighter. Just as a car's headlights grow in brightness the closer they are, so too do stars appear brighter when they are physically closer. Therefore, a less luminous star that is much nearer to us can still appear quite bright (try tapping on Sirius, for example). This is known as 'apparent magnitude' and doesn't tell us much about a star unless we also know its distance. Stellar luminosity, on the other hand, is an intrinsic measure of the total amount of energy emitted from a star. A star of 10,000 solar luminosities outputs 10,000 times more energy than the Sun and will sound louder when tapping the star, regardless of its distance.