I left out these sentences which are more or less the same but I need to know which are possible. Thank you very much.

1) I'm the singer in the music (is musical possible?) band The Boyz.
2) I'm the singer in the band, the Boyz.
I'm the singer in the band called the Boyz.
3) I'm the singer in The Boyz band.
I'm the singer of the Boyz band/ in the Boyz music band.
4) I'm the singer in a music group called the Boyz.
I haven't washed the dishes (not just dishes) yet.
5) I still haven't done an art test today.
I haven't had dinner yet today.
I have already done an English test.
6) I haven’t auditioned for a part yet.
I haven’t studied for my history test yet.
7) I haven’t bought a new math (maths) exercise-book yet.
I have already done a lot of sport today.

1) I'm the singer in the band, The Boyz.

Since bands usually play music of some kind, there's no need to use "music."

2) OK

3) None are OK.

4) OK

5) I still haven't taken an art test today.
I haven't had dinner yet today.
I have already taken an English test.

6) Both OK

7) OK
I have already done a lot of exercise today.

To determine which sentences are possible, we need to consider the grammar and usage of the phrases given. Here's an analysis of each sentence:

1) "I'm the singer in the music band The Boyz."
This sentence is grammatically correct, but the phrase "music band" sounds a bit awkward. It would be more natural to say "I'm the singer in the band The Boyz."

2) "I'm the singer in the band, the Boyz."
This sentence is also grammatically correct. It could be rephrased as "I'm the singer in the band called the Boyz" to make it sound smoother.

3) "I'm the singer in The Boyz band."
This sentence is grammatically correct, but it's more common to say "I'm the singer of the Boyz band" or "I'm the singer in the Boyz music band."

4) "I'm the singer in a music group called the Boyz."
This sentence is grammatically correct and sounds natural.

5) "I haven't washed the dishes yet."
This sentence is grammatically correct and conveys the intended meaning.

6) "I still haven't done an art test today."
"I haven't had dinner yet today."
"I have already done an English test."
These three sentences are all grammatically correct and make sense.

7) "I haven't auditioned for a part yet."
"I haven't studied for my history test yet."
These two sentences are grammatically correct and convey the intended meaning.

8) "I haven't bought a new math exercise-book yet."
"I have already done a lot of sport today."
These two sentences are grammatically correct, but it would be more common to say "maths exercise-book" and "I have already done a lot of sports today."

In summary, sentences 2, 3, 4, 5, 6, 7, and 8 are all possible, with some minor adjustments to make them sound more natural.