Hello nclt,
From your post it appears that you have an issue with the volume suddenly changing on your Mac.
The first thing we recommend is to restart the Mac. If the issue continues, and it is an Intel based Mac, then we would perform an NVRAM reset. "NVRAM (nonvolatile random-access memory) is a small amount of memory that your Mac uses to store certain settings and access them quickly." Reset NVRAM or PRAM on your Mac - Apple Support
If it continues, try starting up the system in what is called safe mode. This will help isolate this issue.
To start in safe mode, begin with the computer powered off. Follow the directions found here, depending on what type of Mac you have: How to use safe mode on your Mac - Apple Support
The computer will take longer to start in safe mode, and the screen may have some distorted video as it starts up. This is normal. Once you are logged in, use the system and test to see if the issue continues.
If everything seems to work as expected, go ahead and restart the computer normally and test again. Starting in safe mode forces the computer to run some disk checks similar to First Aid. This can resolve some issues.
Have a good day.