How do you think ratings work?
Broadcast television is receive-only. Even your digital cable box doesn't send information back about your viewing habits, even though it technically has an upstream channel (for the digital guide and on-demand and PPV features), that would be violating your privacy.
In the US, you get offered to be part of a "Nielsen family". Nielsen is the company that records ratings in the United States. They give you a special cable box/DVR/etc that sends your viewing habit information back to Nielsen over the telephone line (or now I imagine, over the Internet). I have *no* idea how Nielsen tracks radio ratings, but I imagine it's something similar - special receiver. The idea is that because it's opt-in, but you have to be selected randomly to have the opportunity to opt-in, they form a representative sample of the entire population. Ratings are just an estimate.
In Canada, a company called BBM (no relation to Blackberry) does our ratings. The principle is the same - it's just a representative sample - but the technology is different. Here BBM gives you something called a "Portable People Meter". It's a pager-sized device that has a microphone and a little processor, and shows - TV, radio etc - that want to be tracked insert inaudible codes into their audio streams. The PPM picks up these audio codes and tracks what you heard that way. At the end of the day, you sync your PPM with a device at home that charges it and uploads the data to BBM.
So anyone wearing a PPM will not count a CP24 broadcast that is muted.
And now you know.