I certainly agree with gashcan that "... so, sit back, relax and enjoy the flight" is pointless.
This question crops up regularly in one forum or another and the SEARCH facility should provide many more threads from history.
When these threads come up, I always make the same point: Audibility. When you are speaking to ATC, they are listening through high quality headphones in a room that is (hopefully) fairly quiet.
We are sitting in a room that has air noise from outside, probably some engines as well. On the inside people are chatting, air-con is blowing and a meal service might be in progress. Headphones that are in service are listening to privcate music OR you just interrupted the film soundtrack!!
So .. speak clearly and slightly louder than you might otherwise have done. Often times, I have heard a voice whispering away in the speaker above my head (or offset by one row if the original specifier of the a/c did not put speakers above every row!) and I hear nothing of what is said.
Of course, if you are in a turbo-prop then Shout! So, on behalf of the crew, I hope that you have a nice and relaxing read of this thread and that we will see you on PPRuNe Forums again very soon. If there is anything that you require, then just press the button marked Moderator.