What if the security at the event is the police? Freedom of speech, like all other rights, is not absolute. The cut off usually comes when one individual exercising their rights starts to infringe on another's ability to exercise their own rights.
Preventing people from communicating is infringing their right to free speech, so there is certainly a balance there to be found. Note this does not include speaking against people, or forming an opinion based on someone's speech, all of which are perfectly legitimate.
Is it OK for a group of people to stand in front of an anti racism speaker and just scream through megaphones every time they start to speak? No content, no opposing arguement, just literally screaming so the opposition cannot be heard? I certainly so not think this is acceptable.
You're still not getting what "freedom of speech" is!
http://en.wikipedia.org/wiki/Freedom_of_speechArticle 19 of the ICCPR states that "[e]veryone shall have the right to hold opinions without interference" and "everyone shall have the right to freedom of expression; this right shall include freedom to seek, receive and impart information and ideas of all kinds, regardless of frontiers, either orally, in writing or in print, in the form of art, or through any other media of his choice".
Everyone gets the right to impart information orally without (state or political) interference. It is NOT a right to a quiet podium and a keen audience at all times.
Preventing someone from expressing oneself, e.g. by shouting through a megaphone, should indeed get you ejected from whatever forum the speaker is at, and possibly land you with an offence under the Public Order Act (in this country), but it's not a violation of the human right.
The human right is there to make it e.g. illegal to arrest someone for what they say or think, BUT (as you say) with some limitations. If there are grounds to prevent someone speaking (wikipedia again has a nice little list: libel, slander, obscenity, pornography, sedition, hate speech, incitement, fighting words, classified information, copyright violation, trade secrets, non-disclosure agreements, public security, public order, public nuisance) the government may do so.