removing text typing sound from a slide

Don't be so quick to assume something is a bug just because you don't know the workflow.

Typing text objects are a bit of an unusual case in Captivate because they are locked into the slide more deeply than other objects, and hence have to be dealt with somewhat differently.

Right click on the typing text layer, and select Replace with Text Animation from the context menu.

Once you've converted the typing text object to a a text animation, THEN you can delete it. You might have designed Captivate to work differently, but then you might not have thought of giving the users the option of converting typing objects to text animation.