I don't know if this is it, but with my PC, the speakers will randomly "Shhh" if my phone is nearby. Sometimes it happens when I receive a text/call. It could be some kind of interference similar to that .
My son just set up an old Emerson portable radio/cassette receiver system at work to replace a similar unit that died due to a electrical storm from the other day. He has his mp3 player attached via a corded in jack. He had his cell phone just behind the unit and when he received a call the music would cut out and the speakers became a god offal moaning ring tone.
So, yes cell phone and other devices can cause interferences.