Why is the audio of my MacBook is becoming scratchy?
A vacuum cleaner can damage the small cones attached to the magnetic cone of a dynamic speaker, like in your MacBook Pro.  Similarly, a can of compressed air can damage the diaphragm.  One reason for buzzy, distorted sound is damage to these speaker components.  

https://www.ifixit.com/Store/Mac/MacBook-Pro-15%22-Retina-Mid-2014

The above iFixit website (with whom I have no affiliation) describes how to tear down your Macbook Pro to see if your speakers are damaged.  They also sell replacement speakers.