Question

why are the walls of halls and auditoriums covered with soft material such as curtains?

Answer 1

This is too prevent the sound to get disturbed by getting echo. Soft materials like curtains observe the sound and does not let it reflect back. This provides a good quality of sound.

Answer 2

This is done to avoid reflections of sound.These are good absorbers of sound. The curtains absorb most of the sound falling on them, and hence do not produce echoes.