My earpiece speaker is not working.

Hi, I have two iPhone 6's where the earpiece speaker does not work. I have changed iPhone screens multiple times on various iPhones so I have knowledge of the procedure. However, after trying to replace the earpiece speakers on these two iPhones with multiples replacements including purchasing replacement screens where they are already pre-packaged/built so all I have to do is connect the cables to the screen, home button ports etc, it's still not working. Everything else for the connection works like the proximity sensor. It's just the earpiece speaker that does not work. Any help? Thanks.