No, sadly I don't know how to detect my subscriber amount on youtubeCan your program already detect the current number subscriptions are made on youtube? I suspect that is that hardest part -- learning the Youtube/Google authentication and APIs. You could go with some page scraping, but I suspect you will run afoul of YouTube's Terms of Service. Keeping your program running on a schedule, and eventually to play a sound when you hit the threshold is going to be the easy part.
The reason I posted here "Components and Controls" is because I didin't found youtube api or something like that.Were you posting here under "Components and Controls" hoping there was a ready made component or control for you to just drag and drop into your program? If so, I don't think one exists for such a specialized and specific need.
Okay, let's step back even more. How much C# programming do you know?
Or are you simply asking people to write the code for you? If so, your title should have been "Need complete program to say 'yay' when I get 15 subs on youtube", which would imply that you need somebody to provide you the code, as opposed to your current title which implies that you just need help understanding the techniques needed, but you'll do your own coding.