Why is the audio of my MacBook is becoming scratchy? Okay, this is very weird. I've a MacBook Pro Retina 2014 model. It's all going very well. Sadly I've to use it in a dusty area most of the time. I've noticed the quality of the audio (any sound, iTunes, YouTube, etc) is getting really really worse gradually. Lots of buzzing and screeching and it's not as loud anymore!

I usually clean it by vacuuming around the keys and under the screen. I did look up some results online and couldn't find anything useful! 
Any help?
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.