Q: iPhone 4 - no sound either way when calling.

If you can't hear a person on a call or voicemail or if the sound isn't clear on your iPhone - Apple Support

If those suggestions do not help try a System Restore. First backup your device using iTunes. Also import your photos to your computer and copy all important data. Try restoring from backup first and testing it. If it does not help, you may have to restore as New and reconfigure from scratch as the backup could be corrupted. For this reason it is important to have your photos and data saved separately from the backup. Here are the steps for a Restore:

https://support.apple.com/en-us/HT201252