It is explained in the passage that only a very small percentage of the sound waves ----.

Sounds produced by continuous vibration tones are
spreads waves of compression through the air. Where
there is a solid boundary such as the walls of a room the
sound waves are reflected so that the sounds within the
room are prolonged beyond what they would be in the
open. The sounds produced by the voice or by a musical
instrument then reverberate through the room after the
actual tone production has ceased. When the sound
waves strike the walls some of the sound energy travels
on and is either absorbed in the material or may
penetrate to the other side; but with the usual hard,
unyielding walls of which most buildings are made, more
than 90% of the sound energy is reflected back into the
room at each impact, so that some time must elapse
before all is spent. It is this reverberation which, in its
excess, is the prime cause of the faulty acoustics of
many pre 20th century buildings.