iPhone 6 Plus Sounds Stopped Working
If you can't play music either or listen to voicemails (to check the receiver and the speaker) then you have a hardware or software issue.

If you can play music and listen to voicemails, then you have a software issue or a configuration/setting mismatch with what you want the device to do.

Hardware issue - usually starts with eliminating software issues, but in this case observing the headphone port for damage might be a great quick fix.

Software issues, back up the device and test after restoring it and verifying the settings.