Keynote is hit or miss when playing sound to images as there isn't an internal sync system built in to Keynote. Simple short duration presentations are not an issue. Close all open applications except for Keynote and have a good amount of RAM in the Mac are simple things that will help playback.
For self playing presentations that require precise timing, use a video editing application to create a Quicktime file. I regularly create image content in Keynote, export to Quicktime then combine the images and sound in Final Cut Pro. iMovie or a dozen other applications would work also.