So do most PC's come with microphones nowadays? Don't laugh, I honestly don't know haha. Mine is a non-top-of-the-line desktop from summer 2004.
I've never had a computer with a microphone. My first PC was a TANDY. True story.
I don't know about most, but more and more do. But most of them aren't ideal for speech recognition, since you need the mic to be close to your mouth, and the mics that are built into laptops or come with webcams are the ones you sit a bit away from.
With that said, you can get a perfectly acceptable headpiece mic for probably 20 bucks.