From my own personal experience supporting thousands of our organization's Macs, the speaker is the weakest link here. I've never seen any other speaker from any other model laptop fail at nearly the same time or as often as the speakers in the MacBook Pro 2013-2015 Retina models (both 13" & 15"). The only conclusion I can come up with is that these speakers are just deteriorating due to old age since they all began to fail at nearly the same time for each model year group. The thin material used in the speaker which produces the actual vibrations which you can hear can degrade over time....it seems the material Apple used in these particular models is degrading at a much faster rate than other models (or maybe there was a defect in manufacturing that is only now showing up as the material degrades).
I have not been able to locate any brand new speakers outside of Apple for these models (all of the 13" 2013-2015 MBPro use the exact same speaker, and the 15" models 2013-2015 all use the same larger version of the speaker). If I'm correct, then you would want new speakers manufactured more recently since even a brand new unused speaker made in 2015 may still have the same issue since it would be 8 years old now.
You can try another used speaker, but I wouldn't expect it to hold up for long either (you never know though). If you have the 2015 model, then see if Apple can replace the speakers for you assuming the speaker is still available since the 2015 model is now "Vintage". Just how much money are you willing to risk for such an old laptop? Probably time to use headphones or external speakers.
Sorry I don't have better news for you.
