MacBook Pro speaker

I have found the speakers in the Retina model MBPros from 2013 to 2015 tend to become distorted due to aging....these are the only laptops where I have ever seen this happen. You will need to replace the speakers. Replacing the speakers on the 13" model is relatively simple (the 15" model is not easy), but you must be extremely careful when disconnecting the speaker cables from the Logic Board since some of the connections are tight and could lift the connector from the Logic Board. Unfortunately most speakers available online will be used speakers so they may not be the best quality since they are just as old as the speakers you are replacing. Just remember to disconnect the battery from the Logic Board before connecting or disconnecting any internal components.

This laptop is considered "Vintage" by Apple, so most likely only an Apple Authorized Service Provider who repairs computers in their own shop may able to perform an official repair, but only if the part is still available from Apple (FYI, the speakers are the same as used in the 2015 models).