Unwanted Audio
Sounds like the original problem may have been solved; however, I'd like to add a note that has not been mentioned. I have learned from experience that speakers may emit sounds from strong radio sources if they are nearby, even with nothing powered on. I have heard this from emergency vehicles passing my house, briefly. It has happened at two different houses and on different speaker systems. It sounds like the problem here was TV ads' (or maybe radio) audio being picked up, not ads targeted to computer users, which makes me suspicious, especially as commercials are often louder than the programs. If I recall correctly, detaching the speaker wires from the speakers will stop the problem, so I suspect speaker wires are acting as antennae. If this problem is on external speakers, I would try using shielded wire to connect them. Using twisted-pair wires might also work as this rejects "common-mode" interference - but use something bigger than phone or internet cable for speaker wires. Also, you can try moving the computer setup.