Hmm two threads for the price of one 8)
First thing I'd do is find a switch that is not going to annoy other people if you turn it on and off frequently and use that as your "test signal".
I'd then move the audio cables away from the power cord and "test" to see if it was just cross coupling.
Then if that has not solved it if you have a handy "filter mains lead" such as thos you see in computer shops with four or six sockets in plug that in between the wall socket and the computer and "test".
You may find that the mains side filter in the PC Switch mode PSU is starting to break down, or that the earth/chasis connection in the mains cable / wall socket has gone dry or worked lose.
Sadly it's a case of "walk the line" testing till you stop it or get a change in charecteristics of the noise to localise the fault (unless you have the appropriate and expensive test gear to just plug in and run).
But onto the second more interesting thread
There are various ways cell phones transmit and receive the data that is the phone call these days (that is most analog systems are long gone).
The main charecteristic distinquishing them is not the RF band they use but the "signal envelope" and how easy that is to detect.
In some cases (GSM) where the RF signal envelope (not the RF itselfe) falls inside the audio band all that is needed is an unshielded audio amplifier input. The RF signal envelope is "envelope detected (just like any simple AM radio detector) and the resulting audio signal gets amplified as audio and makes a grating grunging noise in the speakers. Thus a very cheap AM radio tuned to a moderatly weak station or even blank part of the band will pick up an adjacent GSM phone.
However some mobile phones (CDMA) effectivly use a continuous envelope signal and this has to be done in a different way. One is the simple "bug hunting" diode detector. Put simply you have an antenna with a parellel tuned circuit at the center of the RF band used. Any signals will cause an RF voltage to appear across the tuned circuit. A suitable signal diode will convert this RF signal into a DC signal and this can be used to drive a simple meter via a lowpass filter and amplifier.