Captivate 7 Audio Issues

Hi,

Created a Captivate software simulation with separate audio recorded on each slide.  All sounds fine until I publish as MP4.  I then get a jittery or echo type sound on just some of the words on some of the slides.  This changes to different words when I tried re-publishing again.

These are the settings I have chosen in SWF size and quality and I have also turned off Play tap audio for recorded typing in Publish Settings.  Please please can anyone help as this is driving me mad!!  Thanks,  Lisa