Use this guide to replace a broken speaker in your iPhone 6.
This guide instructs you to remove the front panel assembly; this is intended to prevent accidental damage to the display cables while you work. If you feel comfortable supporting the display carefully while removing the speaker from the iPhone, you can skip the display removal and go directly to the speaker removal steps.
Please note that distorted sound from this speaker might also be an indication that the grille is blocked. In that case, follow this guide and use IPA and a bristled tool or compressed air from the inside of the phone to clear the holes.