Q: My volume changes without me even touching the button and the volume down button doesn't work properly. Only just bought this laptop yesterday, brand new.

The symptom you're describing could be either hardware or software related. The troubleshooting steps as follows are to start isolating the issue on both ends.

From a hardware perspective, start with an SMC reset and an NVRAM reset. You can also run Apple Diagnostics to see if it picks up any immediate hardware issues.

After that, if the issue persists, try things out in Safe Mode and in a Test User. These will start looking into software issues.

Of course, another option, if you want, is to return/exchange the machine for another one. The return/exchange policy with Apple is 14 days. Just to keep that on the back burner.

Let us know the results!

Sound Levels - Troubleshoot