"quiet music" refers mainly to the volume. But you may choose certain genres if you wanted to listen to music quietly e.g. easy listening, some classical music. So in context, "quiet music" could also give a general indication of genre. "soft" or "light" music refers more to the genre. "soft" suggest relaxing, soothing, perhaps nighttime music e.g. slow piano music. "light" suggests it is not serious. It may make you feel happy, a bit lively and could cheer you up. A good example is the whole genre of classical music called "(British) light music". https://en.wikipedia.org/wiki/Light_music This kind of music is often based on folk tunes or was music used for films. See also https://www.youtube.com/watch?v=Ceqpno_zr6c Soft music and light music are overlapping definitions, I would say.