I might be wrong
But i think the volume change is just because of the sound compression in the adverts. Adverts are rubbish and more compressed than normal TV programmes etc. Because it is more compressed it makes everything louder.
( Herman Blume is unique, just like everyone else, Thu 15 Apr 2010, 17:04, 4 replies)

It certainly applied
before the age of digital TV, where compression was not an issue.

I think in fact, it dates from before mute buttons and has just never really been re-thought.
( Ugi , it's a game of give and cake, Thu 15 Apr 2010, 17:07, closed)

No, it's done deliberately.
There was a case recently where (may have been ITV) a channel was fined for having the ads too loud. There are guidelines to adhere to for volume!
( Blind Meach Cautiously optimistic, Thu 15 Apr 2010, 17:11, closed)

don't about digital TV back home
but here in Spain, if anything it's got worse since digital TV came along - and now we're almost all completely switched off from analogue as well.
(bamboozled Can hear you getting fatter, Thu 15 Apr 2010, 17:31, closed)

Yep it's compression
But in the old analogue sense - every sound is it the maximum possible volume allowed by law rather than having the natural variation that the programme does. So yes, on average ads are much louder but their peak volume is not higher than the programme.
( pixel_fucker is going to need a bigger brush, Sat 17 Apr 2010, 21:27, closed)