How would one go about changing the volume on one PC(x) from a separate computer(y) over wifi/internet? Is this possible using windows.h? I'm pretty sure it's possible, but how would I go about controlling the volume of x from y? In other words, how do I use windows.h on x, from y.
Not directly possible. What you can do is break the application in two pieces:
1. A piece MUST be installed on target computer, adds an entry on windows firewall on a specific port and listen to it ; when receive a well defined command on that port it change the volume; the protocol can be anything you want
2. A piece can be run anywhere on the internet , connects to the target computer using IP and port of it and sends specific commands which the server understands.
There are various security issues which will not allow you to do this. You can try with teamviewer but you will not be able to change Drive Volume of the Main Drive i.e C drive.