today's new phrases

(So you know, this sounds sarcastic, especially with "could it be" i.e., that it is obvious that this is the reason.  But the latter part sounds slightly strange because "sacked" means fired, but that doesn't necessarily mean staying in the house.  In other words, "because you haven't been to work in two weeks" sounds more appropriate.)
-I am sure to get an offer from the company I applied to last month.
-I am looking to join a creative writing class in a local university.
-'I have a sore throat' 'if you will shout so much, I'm not surprised.' (expressing disapproval of something someone has done)

"If you" sounds strange here.  Especially with "will".  You could say "If you shout that much, of course you will get a sore throat".  Or with "I'm not surprised", just "You shout so much, I'm not surprised" sounds better.  I.e., ."if" is more about whether or not there is shouting.  So you want to use "that" with it to imply degree, maybe like ほど?  And "something one has done" implies something one has oneself, personally done, not someone else.
-I feel bored of my job. I'm stuck in a rut. (マンネリしている)

I looked that up.  Apparently マンネリかする means "getting stuck in a rut" or "becoming stereotyped" but its different from the English word "mannerism" which means more like 性格の特徴 especially with body language or body gestures.

I would be grateful if anyone could correct these sentences, thank you!