Aside from the three songs written for the Yule Ball in Goblet of Fire all other songs in the Harry Potter films are instrumental only.
J.K. Rowling has stated numerous times that she will not write another Harry Potter book. She is working on other projects which she states will not have anything to do with fantasy as she doesn't want to borrow too much from Harry Potter.
Because the band Simple Plan wanted it to be a simple plan to become famous and the movie showed how it looks like a Simple Plan well accomplished.
They got their name from the movie "A Simple Plan":)
The reasons are quite clear in the book. Briefly, this was part of Voldemort's plan, to get Harry Potter into his power, use his blood for his "rebirth", and then kill him. This last part of Voldemort's plan failed.
The books contained many continuity errors when it came to the locations of things inside Hogwarts. The Harry Potter Lexicon and the Harry Potter Wikia are the best places to find the most information but there is no one floor plan to Hogwarts.
The final chapter is called "A Flaw In The Plan"
pg. 736 in the 7th book chapter, "the flaw in the plan"
No there is not. plan and simple :)
They were discussing with Voldemort about their plan to kidnap Harry Potter at the Quidditch World cup. Barty Crouch Jr. was telling Voldemort he would not let him down.
The level after Dobby! Is called The Thief's Downfall.
Rowling does not plan to write any more books about Harry Potter and the wizarding world. She once mentioned writing a guide to the Harry Potter books (including characters, background stories, etc). She will, however, write more books about other subjects.
No, there is only one Harry Potter series. It would breach copyright laws to create another series under the same name. J.K. Rowling does not plan to release anymore books in the series.