What is the difference between an audio cd writeable and normal one?

Nooooooo no no no no!!! You're all missing the point here.

"Audio" CDRs are CDRs with additional info on them used by consumer hi-fi CDR units. They are not designed for better CD audio. The reason they sound harder is because they're actually *lower* quality -- because they're designed to be written to at lower speed than computer CDRs.