Audio problem: McKay is talking to Sheppard about the differences between himself and Rod, and at one point says that maybe Rod screwed up and was chosen for "a one-way ticket to another universe", yet his lips form the word 'galaxy'.
Audio problem: In the scene in the hallway when Sheppard and Teyla are dressed in hazmat suits and talking to Dr Weir via the intercom, you can see a reflection in Teyla's helmet of Sheppard's mouth moving, but there's no sound. (00:23:45)
Audio problem: Teyla's audio sounds way more high pitched then normal, either dubbed or a post production mistake. (00:06:00)
Chosen answer: Because the ZPM allowing them to dial Atlantis was being used at the Antarctic defense outpost. They couldn't risk making Earth vulnerable to attack from Anubis by depleting it with repeated dialing to the Pegasus galaxy. Once they knew Atlantis was in serious danger, they had no choice.
Grumpy Scot
ZPM from Antarctic was depleted in first dial, dial in Siege was done via ZPM from Egypt (SG-1: Moebius Part-1/2).