I would argue that it might be best to keep Woolf's phrasing in its most simple terms as possible.  Part of her gift is that she is able to develop ideas and notions that really challenge one's mode of thought.  Simply assessing the meaning of these words can lead to more questions and greater thought, which has to be one of her end results and desires.  I think that the phrase in question is a powerful one because it speaks to how things in life change.  For Clarissa, the idea of Big Ben's chimes as "a musical" almost comes to represent how there is beauty in this sound, a sound of time passing. Yet, Clarissa then understands that this becomes the critical element of both Big Ben and her own being.  Each sound represents a form of beauty, but also represents something irrevocable.  "The Hours" pass, and within them the struggle to find meaning as they pass.  They move as "leaden circles" whose lack of possession become akin to "circles dissolved in the air."  The passage of time in human consciousness and the struggle to make meaning of such passing becomes one of the driving forces in Clarissa's state of being.  It is also critically important to the theme of the novel in that individual identity is something that is inextricably linked to the passage of time.  The beauty of Big Ben's chimes also reveal something more chilling within them.  This becomes the challenge of time that constructs one's being and what Clarissa reveals in her own understanding of it.