It was/had been so long...
I have read this sentence in a novel:
It had been so long since he had heard anything but the sound of his own voice.
I rewrite this sentence as:
1. It was so long since he had heard anything but the sound of his own voice.
2. It was so long since he heard anything but the sound of his own voice.
3. It was so long since he had last heard anything but the sound of his own voice.
4. It was so long since he last heard anything but the sound of his own voice.
5. It had been so long since he had last heard anything but the sound of his own voice.
6. It had been so long since he last heard anything but the sound of his own voice.
7. It had been so long since he heard anything but the sound of his own voice.

Then what's the difference between sentence 1~7?
Thank you in advance.

Question

It was/had been so long...
I have read this sentence in a novel:
 It had been so long since he had heard anything but the sound of his own voice.
I rewrite this sentence as:
1. It was so long since he had heard anything but the sound of his own voice.
2. It was so long since he heard anything but the sound of his own voice.
3. It was so long since he had last heard anything but the sound of his own voice.
4. It was so long since he last heard anything but the sound of his own voice.
5. It had been so long since he had last heard anything but the sound of his own voice.
6. It had been so long since he last heard anything but the sound of his own voice.
7. It had been so long since he heard anything but the sound of his own voice.

Then what's the difference between sentence 1~7?
Thank you in advance.

Answer

In the original sentence, both parts use the past perfect simple tense. (It had been. He had heard)
PPS is used to talk about an action that happened in the past, and before another action. 
Fictional books are often written in the past tense, so PPS is used a lot when describing events that happened in the past from a character's perspective.

Your sentences make sense, but are all slightly grammatically incorrect, using the wrong tense forms in places.

I'm not perfect when it comes to grammar though, so don't take my words as law!

Answer

What is the reason for needing to rewrite this sentence?

As Adam said below, novels are often written using past tenses, and PPS is used to describe actions that happened before other actions. It's important to convey the sense what happened when/the sequence of events.
Your sentences, although understandable, are slightly grammatically incorrect and don't reflect the sequence of events.