Read http://www.mactech.com/articles/develop/issue_16/034-038_Qui..., especially the part that says:
"Square wave sounds. Unknown to most, the square wave synthesizer never produced
true square waves. It was more like a modified sine wave. This has been corrected.
As a result you'll notice that the Simple Beep sounds different. It can now be
heard as it was originally designed to sound."
What I don't quite get is that the asm code seems to start with a sharp square wave that smoothens off as time passes, but in that video the first sound already sounds very smooth in the beginning.