Translation:The sound of the harmonica is perfect.
"Ο ήχος της φυσαρμόνικας είναι τέλειος" would be correct and mean exactly the same thing. It's probably what I would use in real life too. But be careful to add the ς at the end of φυσαρμόνικα since it's now a different case.
No. You don't hold a harp up to your mouth and blow in; you sit in front of it and pluck the strings with your fingers.
Interesting. Apparently a "blues harp" is a kind of harmonica. I hadn't heard of that instrument name before.
I'm not sure whether plain "harp" would be understood as a wind instrument. Perhaps in proper context.