There are probably some crap audio booster applications somewhere, but it'll no doubt distort the signal more as well. I don't even like the Windows volume sliders on 100%.
I think iPlayer content is just mixed significantly louder than normal stuff anyway. It's much louder than everything else I play on XBMC, including broadcast TV and downloaded content.
I tend to have YouTube on about 40% (with Win at 50%), and I think I might have Vimeo even lower.
But then Jango I put at 100%, and then vary Win's level from 60% upwards (which is now much easier, yay), because they don't have a normalised catalogue (and some songs just sound better louder/quieter anyway).
ITV's player's pretty cool, as it seems to dip the volume lower for the adverts on anything I normally watch. It's a pity they can't do that on the actual TV, as usually the adverts are shouting at me instead.