Pictures are copyright (C) 2008, Peter Marshall
Although there was never any real chance that the noise made by protesters would actually drown out Blair's amplified speech within the rather solid walls of Westminster's Roman Catholic Cathedral, it could be heard in the background as his speech was played in the radio news although no mention was made of the demonstration on those I heard.
Of course all of those entering (except any like Blair himself creeping in - and later out - by a back entrance) were very aware of the protests as the queued outside to have their names checked on the register and go through the metal detectors before being allowed in to the cathedral.
More pictures on line at My London Diary, http://mylondondiary.co.uk/2008/04/apr.htm#sound