1. Rectangles signify this traditional square mindset and way of thinking that may be associated with the older generation and how they may perceive things / music. you can say that they represent the older generation or simply the mindset that rectangles=angularity and a stubborn/old-fashioned way of thinking
2. the forming of fluid forms using rectangles then shows that even the traditional can be re-interpreted and that nothing is definite. or perhaps, even the 'traditional' may be something new
3. the artistic is commonly associated with free-flowing forms and curves and this serves to tell many mis-guided individuals that no this may not be the case. and never to assume so. of course the forming of these curves in this case serves to be sarcastic to those who believe that the artistic=freeflow curves and forms
4. on a nice/cliche note: many rectangles signify the individuals in the orchestra where each little individual contributes to the whole picture and the creation of a wondrous piece of music. and each individual is unique and painstakingly drawn.