I have a Samsung Galaxy S5 from Verizon and I have been experiencing similar issues with an audio message 'Battery Fully Charged' when the battery is 100% charged and the phone is still plugged into the wall charger. I called Samsung and they remote controlled into the phone via a Samsung app, but could not determine where this message originated, or how to disable this audio notification.
I decided to try and uninstall apps one at a time to see if I could find the source of this issue, and stumbled upon a Google Play Store app called 'Faster Charger'. This app, it appears, has a default setting to notify the user every 5 minutes when the battery is 100% charged. I uninstalled this app, and the notifications went away. I then re-installed the app, and as soon as I plugged in the charged and started the service, the messages came back immediately. I have uninstalled the app and do not intend to re-install it again.
Hope this helps!