There seem to be layers of beauty in music that mean that it is both an objective feature of the music, and simultaneously subjective. There are, evidently, accepted and well-established compositional techniques in every era and genre of music (though, whether these are aimed at achieving beauty is another matter). There seems, then, to be a given currency with which the composer works. This does not necessarily limit the freedom of the composer, however, who is able to flout these given ‘rules’, as it were. Though, only by understanding these is one able to understand how to effectively go beyond them.
There is, however, some degree of objectivity in discussing what is beautiful in music. It is difficult to say what it is that makes a piece beautiful, as saying, for example, a tierce de picardie is beautiful implies that it could not be used in a way that is not beautiful, which is evidently not the case. It is perhaps easier to agree on particular musical works which are to be called beautiful, rather than particular compositional techniques, which may be used in ways which are beautiful and in ways which are anything but. Though, again, there would be a range of views on any one particular work, the subjective level. Perhaps the general consensus might be the nearest thing we get to a nod towards the objective level of beauty.