Check the ring/silent switch on the side of your phone. If you see the orange line then your phone is on silent mode.

Mercy68 wrote:

 

Do you have a solution of why I can't use my decrease sound button? It just stopped working.

Try holding the sleep/wake and home buttons until the Apple logo appears.

Try the below steps:

Back up your phone: How to back up your iPhone, iPad, and iPod touch - Apple Support

Restore your phone: Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

If the restore does not fix the volume button, make a Genius Bar appointment or contact Apple to have the phone evaluated. Contact - Official Apple Support

Yes, restoring it to factory settings would erase all your data. Your phone will be like it was when it came out of the box. However, if you back up your phone before restoring you can test the button without any of your data and then restore back to your backup if the issue was not fixed. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

You are welcome.