What must you do to the strings on an instrument to make a louder sound?

Amplifying, or amplification would be common descriptors for the process of making sound louder.

In Musical Instruments, sounding boards are one common practice. These vibrate a larger surface. This applies to piano and violas. A horn or trumpet is a device for matching the impedance of the source to that of free air, and this device is used in the brass instruments(including the digeridoo!) and some wood wind instruments. A car horn and a speaking Trumpet are other examples.

These simple systems may give in excess of 20dB amplification. (100 times).