Why speaker beeps when a cellphone is kept near it? Once I had kept my cellphone near a computer speaker. After some time, a call came over my phone. Strangely, the speaker started  humming and beeping till I received it. Why did the speaker started humming when the call came?
 A: First of all, the effect is totally real. Here is an example YouTube video how it sounds (and a recipe how to fix it):

https://www.youtube.com/watch?v=x5ruAZ4Useg

I am actually getting much more melodic sounds from the speakers! ;-) And I have heard the same melodic reactions of PC speakers in many people's scientific talks and even in TV programs. This iPhone example is pretty close to what I intimately know:

https://youtu.be/wZ9itQJ6i28?t=45s

The nearby speakers may even predict, about one second before a text message arrives, that a text message will arrive. ;-) The cell phone obviously needs to communicate with the mobile network at that time. These signals interfere with the circuits in the computer and/or speakers.
The communication is through radio (or even microwave) signals which have a very high frequency and it's not audible. However, the patterns in the radio transmission may contain components that are at much lower, one kilohertz or so, audible frequencies, and those currents are then directly heard from the computer speakers. The wires going to the speakers are basically used as an extra antenna by the cell phone to broadcast the signal.
It seems impossible that this interference occurs directly. So there probably has to be a nonlinear component in the computer that amplifies the signal and picks the audible frequency in it.
Lots of pages about the effect:

https://www.google.cz/search?sourceid=chrome&ie=UTF-8&q=cell+phone+buzz+from+speakers&gws_rd=ssl#q=cell+phone+buzz+speakers+interference

This Electronics Stack Exchange answer is more detailed than mine:

https://electronics.stackexchange.com/questions/32830/why-does-gsm-cause-speakers-to-buzz

A: According to this article (which links another article that has since become a stale link), the issue is that during ringing (and other communications between the phone and the tower) there are pulses of (relatively) high power RF transmission by the phone.
Normally, this should not matter. But when an RF signal interacts with a non-linear circuit element (for example, anything with a diode in it) it is possible that you detect the "power envelope" of that signal. This is of course how the "detector" element of many radio receive circuits works.
Many computer speakers contain an amplifier - making it well suited both to detect and amplify the RF power waveform, and making it audible. According to the article linked, the conditions are:

The type of interference can occur if the following things happen
  together:
  1) a pulsing radio transmitter,
  2) with relatively strong power,
  3) in very close proximity,
  4) to a non-linear circuit element.
The non-linear circuit element is usually some sort of solid state
  device such as a transistor or diode. If the non-linear element is
  subjected to a strong pulsing radio signal, it will act as a rectifier
  and "detect" the pulsating waveform, i.e., convert the pulsations from
  a radio frequency to an audio frequency (if the pulsation rate is in
  the pass-band of audio frequencies.) For example, a hearing aid
  consists of a microphone, an audio amplifier and a small speaker. If
  a strong pulsating radio signal impinges upon the first transistor
  amplifier stage, the transistor will be driven into its non-linear
  range and detect the pulsations. If the pulsation rate is in the
  audio frequency range, the rest of the hearing aid amplifier will
  amplify this and deliver it to the speaker, to the great annoyance of
  the hearing aid wearer.

I have also heard it said that the signal you are picking up is actually the power being drawn by the device - there are significant (low frequency) fluctuations in power that result in near field magnetic induction. If that is the case, then RF shielding will not help. I may try to do some experiments and will report back...
