Aside from some words most of us do not encounter in our every day lives (like lieutenant), I really doubt how much the media affects the way we talk. Studies have shown that the way native English speakers in Toronto talk has not changed significantly since the Second World War. Sixty year olds and sixteen year olds, for the most part, speak with the same accent. It's in the 70+ category where we see a change. That change is called the "Canadian shift" - the last major change in the way Canadians West of Quebec speak.
The Canadian shift occurs (theoretically) because the caught-cot merger created a gap in the vowel chart. To fill the void, our front lax vowels became retracted/lowered (linguists do not agree on exactly what happened). Of the North American dialects that saw a caught-cot merger, each coped with the void in a different way. The way we filled the gap stands in opposition to the Northern Cities Shift. However, the shift moved in the same general direction (independently of each other) in both California and Canada. So through some great linguistic accident, we now sound more like Californians and less like people from Buffalo.
The Canadian shift appears to have started in Toronto. It was certainly complete here far before it was anywhere else in the country - as I said, at the time of the Second World War. In comparison, the shift is much more recent in English-speaking Montrealers (becoming complete only in the late 20th Century). In Winnipeg it's just started up over the past decade. The largest city in a dialect area is often where linguistic innovations occur, spreading out to other large cities, then to smaller cities, etc. There's nothing new to this.
How long Central/Western Canadian English will remain a homogenous dialect where the same sound changes take place (although staggered over time) is a mystery. There are already signs that it's starting to split up. Young people in parts of BC have lost that distinctive Canadian raising. Even that debate amongst linguists as to whether the Canadian shift is a retraction or a lowering of front lax vowels has been answered with "both, as speech in different areas diverges."
It's difficult to get across to people that we talk differently. People refuse to believe they do the whole Canadian raising thing, but it's pretty strong across the board here. And if it's difficult to convince people here that they say "aboat the hoase," I don't expect many people here to believe that when we say "everyone," the first syllable sounds to non-Canadian ears like the first syllable in "average" (that's not to say it sounds like how we say "average" though). To someone without the shift, my "I'll bet everyone here that that dog is a bit yellow" would sound like "I'll bat* a*veryone here that that dog ez a bet yallow" (* the "a"s here represents the [æ] sound - the way we say "bat" is also affected by the shift, the "a" becoming the [a] sound, like the "a"s in Spanish). If you were born/raised here and English is your first language, you probably speak like this, even if you don't realize it. I remember I didn't realize I said anything strangely until a recent immigrant friend asked me why I always said I was going to be late for "anglish class."
And, to anyone who might be a linguist, sorry if I completely butchered explaining the Canadian shift - I realize some parts of it (especially it's relationship to the cot-caught merger) can be controversial.
