Audio increasing automatically

I have an issue with my audio. The audio increases by itself. No water damage since the laptop, is never near any liquids nor in a humid environment. What may be the cause of this audio issue, hardware or software? If else, how can this issue be resolved.

OS X El Capitan

Version: 10.11.6 (15G1004)

MacBook Air (13-inch, Mid 2013)

Processor: 1.3 GHz Intel Core i5

Memory: 4 GB 1600 MHz DDR3

Graphics: Intel HD Graphics 5000 1536 MB