Well from what I've heard the internal speaker for the Mac Mini is crap, but I have heard mixed reviews that it can be good for music and it's a decent speaker as well. So I really don't know.

External Speakers shouldn't have to be made by Apple to work. Do you still have the box for the HP stuff? If so, it should say what it is compatible with on the box. I am almost POSITIVE that would be compatible with a Mac. I have a new Macbook, but I haven't had to attach external speakers as it has decent built in ones.