Problems c narrations becoming out of synch with animations

I am having trouble with PPT 2010 and a 38 min presentation where after about halfway through the presentation, the audio narration which plays across slides becomes out of synch with the animations. This appears to be particularly the case if the user uses the slide bar to manuallly progress the slides. I have seen other posts about using Kiosk view and am wondering if that helps. Any help would be much appreciated - a wonderful product and thanks!

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I guess it happens due to manual advance of the slides. If the slides are played automatically by timer you shouldn't experience such problems.
In order to get tightly synchronized sound to animations, please add sound files per slide using iSpring import audio tool or record the narration in iSpring through the slides.

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