Não é um problema novo: palestrantes do PC também pegar rádio
This seems to be a common example (to judge by a Google search) of radio frequency interference or RFI, though it's not something I've heard on a PC. The speaker wires may be acting as an aerial and picking up a radio signal, which you then hear from the speakers. The loudspeakers may also be implicated: some models seem to have better shielding than others.
You could try moving the speaker and mains cables, and folding up (rather than coiling up) any spare bits of wire, or changing to shorter, better-quality shielded speaker cables. If that doesn't work, try adding some ferrite rings to the speaker and mains cables. [...]
Basicamente, os cabos das caixas acústicas funcionam como uma antena e captam o rádio normal.