Make sure the factory screen protector is taken off, and that an aftermarket screenprotector does not cover the notch at the top, as that's where the speaker is.
When you get a new phone or wipe your phone, do not re-install your apps from your backup. Contacts would be the only thing I would do. Download everything fresh from the Play store. Seems my girlfriends backup was corrupt.
Before wiping my phone, I took a straight pin and carefully but thoroughly cleaned the speaker. I'd tried cleaning it before with a cloth as suggested in this thread to no avail. After trying this second method, I can hear perfectly. Glad I tried it