Everything except accidentals stays in effect until it is changed. Put a space between notes. If no notes are specified, the sound plays for its natural length.
While sound is playing, HyperTalk scripts go on running and all tools and buttons continue to work. If your stack is on a 3 1/2-inch disk or an HD20, sound will be scratchy whenever HyperCard reads the disk.
You can also specify a note value with a number rather than a letter: 60q is Middle C. 61 is C#; 59 is B, etc.