1. "It's true, it's called living" - life flashing before our eyes when we die. TP for sure.
2. "The pen is very sharp" - and the sword is very small? TP probably too.
3. "A fish!" Weird enough for a Pratchett quote
4. "But, my dear man, the world appears to be ending" Could be, Discworld's been on the verge of ending so many times.
5. "even those members of it who are dead" Has a certain TP turn of phrase to it, inclined to believe it's one of his.
7. "Thud. Splat" The master of readable sound effects!
7a. "glaim gis oteg in der gaing og der" ...and of people talking with something in their mouths.
9. "Death was Nature's way of telling you to slow down" Also very TP like but feels like a red herring.
10."Look out, he's got a gnu!" Definately Pratchett, although not Discworld I think. Johnny Maxwell?
11. "like a homocidal hatrick" confirmed as one of the answers.
I'd say it's either 4, 7a or 9 on the basis that 3, 6 and 7 are so obscure they could be in any book
. Can't make out what 7a's saying so I'll go for that one.
Edit: just read the previous comments, changing to 4 since either 4 or 1 is confirmed to be the other wrong one. So I'm claiming it's 4 and 11!