How do you fix the crackling noise out of the iPhone 6s Speaker?

Hello, I need some help!!

I purchased an iPhone 6s Smart Battery Case, the first charging case Apple has created, for my iPhone 6s. This case has been the best one I have ever had!...until a few days later. I was going to bed, and I was getting ready to sleep. Before I go to bed, I plug in my iPhone with its charging case on, so it's fully charged in the morning. Anyways, when I plugged in my phone, I noticed it was lagging and the volume switch was not working. So, I restarted my device after unplugging it. My iPhone booted back up, and the lagging was worse. But this was NOT the bad part. A crackling sound has been coming out from the speakers! A loud one!

To sum things up, I am going to list bullet points of what's been happening to my phone for the last two weeks:

• Crackling sound out of the speakers (stops then comes back with and without the phone case, but happens all day. Used to be able to recreate the sound, until the charging case was permanently off the phone for a few days. Sound has NOT stopped)
• Volume buttons and toggle switch, before the crackling noise reoccurs, stop working. No menu pop ups, no response
• Vibrations are either weak, unresponsive, or instead making a click noise. When the crackling noise stops, the vibration doesn't come back till 1 minute later.
• No sound from the speakers

Now, I am going to sum up all of the things I have done already:

• Restored the device from a backup
• Restored the device as NEW. After this, the sound got significantly worse, so I had to revert back to the old back up.
• Tested to see the amount of times the sound occurred with/without the case on
• Online chat with Apple Employee(s)
• Took phone to Genius Bar - the sound did not recreate itself while at the store.
• Updated my iPhone to the laste version of IOS

I really need some help!! I am very concerned about my phone, and I know nothing about these crackling sounds!

Thank you!