"Nose bleed seats" are those in the highest part of a stadium, or venue, furthest away from the music, or action. They are therefore the cheap seats. The humorous idea being that you are so high up the altitude causes you to have a nose bleed.
And why did he say (Now) I'm smiling.. while you (Were) clapping
Why were and not 'are' ? Because he is speaking in the pass tense. Are would refer to the present tense.
If I wrote something wrong grammatically or in the spelling please correct it (for) me :)