Amplification of Sound
The movement of the strings themselves is not enough to make any
sort of loud sound waves. Sound waves are simply compression
waves of the air, and the amount of air that the strings
themselves can vibrate is relatively small due to their small
surface area. So in order to amplify the sound, a larger surface
area needs to vibrate. This is where the body of the violin
comes in. The top of the violin is made of slightly more
flexible wood that vibrates easily. The waves from the strings
are transferred to the body through the bridge which compresses
and expands the volume of air inside. This causes compression
waves to emerge out of the f-holes which are much louder than
those generated simply by the strings.
http://hyperphysics.phy-astr.gsu.edu/hbase/music/violin.html