Transformers
The sound you're hearing, is likely caused by transformers. It's a common problem, and a normal phenomenon.
Most wall warts consist of a transformer to reduce the voltage, and a rectifier to convert alternating current to direct current. So anywhere there's an AC adapter, there's likely a transformer.
This document from Federal Pacific (PDF), explains the phenomenon that causes transformer hum well.
Transformer noise is caused by a phenomenon called magnetostriction. in very simple terms this means that if a piece of magnetic sheet steel is magnetized it will extend itself. When the magnetization is taken away, it goes back to its original condition....
A transformer core is made from many sheets of special steel...
If the extension and contractions described above are taking place erratically all over a sheet, and each sheet is behaving erratically with respect to its neighbor. You can get a picture of the moving, writhing construction when it is excited. Of course, these extensions are only small dimensionally, and therefore cannot usually be seen by the naked eye. They are, however, sufficient to cause a vibration, and as a result noise...
Loose Connections
If you're hearing the buzzing from receptacles (sockets), it could be caused by a loose connection. If a connection in the receptacle is loose, there could be arcing taking place. Arcing could definitely cause an audible sound. However, arcing would also generate heat, which could lead to a fire. It's best to contact a local Electrician to come have a look. If you're familiar and comfortable with electrical work, you could check the connections and/or try replacing the receptacle (socket).
