Final Answer:
The sound high in the air is the murmur of maternal lamentation.
The hooded hordes swarming over endless plains are stumbling in cracked earth, ringed by the flat horizon.
The city over the mountains is Jerusalem Athens Alexandria Vienna London.
Step-by-step explanation:
The phrase "murmur of maternal lamentation" is a metaphorical description, suggesting a sorrowful and maternal sound coming from above. It reflects a poetic and emotive interpretation of an unidentified sound in the air.
The description of hooded hordes stumbling over endless plains in cracked earth with a flat horizon evokes a sense of desolation and hardship. The cracked earth and flat horizon contribute to an atmosphere of barrenness and vastness.
The sequence "Jerusalem Athens Alexandria Vienna London" refers to historical and cultural centers. The mention of falling towers and the phrase "Cracks and reforms and bursts in the violet air" suggest a transformative or tumultuous event affecting these significant cities.