Because when people WANT to hear you , they ask you to " speak up " as in increase the volume.
quieten, hush, quiet, quiesce, quiet down, pipe down This answer was found on the following website: http://www.synonym.com/synonyms/quiet/
to calm down; to be quiet
"Pipe down" means to be quiet or to stop talking. It is often used as an informal way of telling someone to be quiet or to calm down.
Means Father, be quiet
The bartender looked up. Tonight's crowd seemed a tad more unruly than normal. "Quiet down!" he called to the patrons.
calm down, be quiet, and be yourself
Have it calm down and put in a quiet place.
I have been called quiet thousands of times. My parents are very quiet people as well so I guess sometimes it is just hereditary.
Turns down the quiet bits
The amount of air inspired and expired during a normal quiet breath is called tidal volume.
The loudness or quietness of a sound is called its volume or intensity.
In the dark, a scream pierced the quiet night, sending shivers down my spine.