I get that from time to time, but turning the volume up never makes the voices any clearer.
I used to get this guy on a ham radio set coming through on my guitar amp, he used to put out his callsign and wait for calls, and I heard him having a conversation once (this guy seems to have been very local to me judging from where he said his location was).
My computer speakers, I think, are getting taxi signals and other rubbish. It's definitely not air traffic control because I've been in the Edinburgh control tower and heard what that's like for myself.
As for a fix, you could try insulating your speaker wire somehow, stopping it from acting like an aerial. Other than that, I don't think there really is a way - it's electromagnetic interference, something we can't control.