Thank you for confirming,
this indicates a defective speaker in the back. The phone uses a separate speaker for this purpose and ringer, which can fail from age/use, being dropped, etc.
Initially try taking the battery out of this handset for at least 15 minutes.
If no change, try swapping in a battery from one of the other handsets. If it works then it was a bad battery. If no change still, the handset is faulty and would need to be replaced (or just used as-is without speaker capability).
If needed you can order a new handset here, but they are generally only available used because this is an older discontinued model: http://amzn.to/2ctG6wv so it may be more cost effective to buy a whole new system for not much more.
If you'd like instructions for setting up a replacement handset, or recommendations for a new system, just let me know.
My apologies for the news.
If you have any other questions, just let me know. Otherwise, please take a moment to rate my service using the stars at the top of the page; hopefully I have earned 5 Stars. I also invite you to contact me directly with any future support needs at http://bit.ly/1QwLxGh and ask for Nathan.
Thank you, ***** ***** a nice day,