iphone4 has black screen. on/off not working. can't restart. can still hear sounds

Hi there malcontenta,

From your post it sounds like the screen is black on your phone but you can still hear sounds from it. I would use these steps from the following troubleshooting article, named:

Get help with the screen on your iPhone, iPad, or iPod touch

If you see a solid color screen (black, white or blue)

1. Plug in your device to a power source. If you see the low-battery screen when you plug in, let your device charge for at least 10 minutes (20 minutes for iPad) before using it.
2. Reset your device while it's plugged in.
3. If resetting doesn’t fix the issue, restore your device.
4. If you still see the issue after you restore, contact Apple Support.

Thank you for using Apple Support Communities.

All the very best,

Sterling