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

Yes, 1-speed. You know those hifi-separate CD recorders, which you connect to an amplifier etc? They use a special kind of bland with extra identifying code on it, and refuse standard blanks. They are more expensive because a royalty is paid to the record companies for every one sold, although the difference in price is typically only about 20p/30 cents a disk now. They are optimised for 1x and 2x writing, although my Teac, Plextor and LG will write to them at 8x no problem, and the Acer will as well as long as they are 74-minute (the 80-minute ones it coasterises above 4-speed).