Right, you had a question. Yes, you should obtain permission to post videos of speakers. The permission doesn't have to be formal (though we have lots of helpful formal permission forms you can use); you can even videotape the permission instead of having it signed. The key is making sure you get permission for all the rights you need.
The speaker's rights. The speaker possesses three types of rights: (1) rights associated with copyright (the text of the speech and accompanying visual presentation); (2) rights associated with the commercial use of the speaker's name or persona; and (3) the speakers right to sue if the video defames or invades the speaker's privacy. What you're probably most concerned about are the first two. (The third "right" is acquired by obtaining a release from the talent.) If you want specific rights, then the release would say something to the effect "I grant permission to [name of association] to use my videotaped presentation [name of presentation] to be posted and publicly displayed at the [name of association's] website." That's called a limited permission/release because it is only for specific purposes. If you wanted all rights -- that is you can do anything with the video -- you would use an unlimited or blanket permission/release.
Be careful what you wish for. As part of your permission request, you might want to seek a "representation" to the effect of "The text and images used in this presentation are original to me and if they are not, I have obtained permission." That way you may have a better shot at going after the speaker in the event the presentation is riddled with infringements and your subsequent rebroadcast triggers a lawsuit.
Bottom line dept. Yes, you should get permission. No, length of video doesn't matter. And you'll only need permission from the venue if your contractual relationship with the venue forbids videotaping or requires permission.