6. Hack Computer
In many situations it's of the utmost importance to able to obtain information from a computer. You have to get around passwords and firewalls setup by the owner, and that can be a difficult task. The sonic screwdriver is well-known for hacking into computers, and all the modern Doctors are doing it. But the sonic has been hacking computer far longer than most people suspect. Fact is, the first Doctor to use the sonic screwdriver to hack a computer was the Second Doctor in The War Games when the Second Doctor needed to access information of the War Chief's plans from General Symthe's computer. This one is a bit more difficult to explain and is supposition rather than outright scientific proof. But the base science is there. Sound waves can be used to alter particle fields and control the magnetism of objects amongst other properties. Altering magnetic fields can then alter the electron flow of the electricity. The magnetism is also a factor in storage capabilities. Altogether this would give a person the ability to control all facets of a computer. If you knew where to point the device you could control the fields that carry the energy and make the computer do whatever you like. The Doctor usually points the screwdriver at the monitor, not the computer but I put that down to visual staging. Many people consider what they see on the screen as the computer. Science is a very long way from this so this is conjecture on my part on how this would work.