Hello,another unusal problem?
Does this sound like it is a radio as in legal one?
Or some random broadcast from say an illegal station?
Do you have say a cable box or sky connected to your computer?
Do you know of any CB users in your area?
Ferrite blocks/rings can be obtained from Maplins in the UK or RadioShack in the US. Or any computer scrap place - if you don't mind taking one off an existing cable.
Try using a mains filter on the power input of your speakers. You should really have one of these to protect your computer from power surges. That will at least allow your to check if the radio signals are being picked up by the mains wiring.
RadioShack.com - Cables, Parts & Connectors: Component parts: Inductive products
Try earthing speakers or are they cheaper end speakers?
Take a empty tube lets say cardboard tube from a tissue roll or aluminium cooking foil.Wrap the speaker cables around it like a coil and tape them in position.
It sounds silly but it can work.
Or if speaker cables are already coiled together uncoil them.
Make sure none of the cables have a small break in them were copper wire is exposed.I mean ALL cables that are connected to your computer.
Try and insulate the cables?
If you are using a wireless router try and move it to another position?
Also your cables and plug sockets try streching them out away from PC or is your computer close to many cables coiled together and realy close to electrical sockets in wall?
Even having a TV arial lead or portable TV with an indoor arial can cause interferance.