In my experience those timings are very much PC dependent.
What we prefer to do, is to
- cut the audio files so that we have one per slide
- add bookmarks to the audio file and make animations start on trigger > On Bookmark ...

In order to avoid your slide proceeding to the next one too soon, you could add an invisible element (i.e. transparent, or outside the shown PPT frame) and have that appear 'After' the audio file has finished.