Hello Lin,
Happy to help!
There isn't a way to navigate through a slide with just the tab button since some non-interactive elements can only be read through the use of the arrow keys. You can read more about how screen readers are able to navigate Storyline 360 courses here:
As for your accessibility question regarding slide audio, the best way to make slides more accessible is to prevent audio from automatically playing when your learners navigate to them. You can check out my response here which addresses a similar inquiry.
Hope this helps!