The reason why it is said that "music is flowing architecture and architecture is solidified music" is because:

Both music and architecture place creation on proportion and structure. They are all inseparable from mathematical calculations. They not only have a complete structure, but also a unified layout. They all pursue harmony, symmetry, balance and the beauty of lines. However, music changes in time, while architecture flows in space.

Extended information:

The deep courtyards of the Forbidden City are like a lingering, intermittent guqin music; touring the gardens of Suzhou, It is also like a beautiful and moving Jiangnan Sizhu; if the Parliament Building in Budapest is in front of you, what comes to your ears is the melody of the classical music school; while the Palace of Versailles in Paris reminds people of the glow of Baroque music.

This Baroque music is very similar to Baroque architecture. It has a meticulously crafted and gorgeous artistic style. The music embodies an unchanging fixed rhythm and a relatively single emotional color, which is no better than classical music or romantic music. Rich and brilliant. Baroque music gives people the feeling of being in a Baroque building. Its splendid spiral corridors, colorful vaults, gorgeous decorations and jade railings all give people a sense of grandeur and awe, and personally they also feel a sense of grandeur and awe. It seems small and insignificant, and you are calm and don't need to care.