adding mp3 files as a keynote narration

I delivered a lengthy presentation using keynote and separately recorded an audio file. I tried to add an mp3 to the keynote and then record the keynote. I did not speak the narration but expected to be able to time my keypresses to create a version of the presentation with a better audio quality but synchronised. The synchronisation seemed to have worked OK but the audio has produced a booming echo similar to Charlton Heston playing Moses. What have I done wrong or is there a better way to doing this? Thanks

Ron