[joshuaheard]

They probably do it because no one pays attention to the announcements anymore. It's the same reason they (used to) turn up the volume on TV commercials.

Has the author tried asking the flight attendants to turn the volume down?

[chadgeidel]

The volume is not "turned up" on TV commercials. The perceived increase in volume is due to dynamic range compression: http://en.wikipedia.org/wiki/Dynamic_range_compression#Broad...

[TillE]

Which means the commercial is playing at maximum volume. Of course it's not physically changing the volume control on your television, but the source is as loud as it can possibly be.