Welcome to the forums. Open the project to slide-edit view to
view the timeline (normally at the top of the workspace). The
bottom item on the timeline is the audio and the item immediately
above that is the "slide" object. Extend the time for the
slide-object in either of several ways:
1) Move the mouse pointer over the right-end of that
slide-object until it changes to a two-way arrow, then
left-click-and-hold while dragging the slide-object's right-end to
a new (longer) position on the timeline.
... OR ...
2) Double-click the slide-object on the timeline (or on the
filmstrip) to open the slide's "Properties" panel. Then adjust the
setting for "Display Time" of the slide.
Either way, the audio will remain unchanged on the timeline,
so lengthening the
slide duration will create additional time
after the audio completes play.
Have a very nice day!
.