I found these allophone recordings: http://little-scale.blogspot.it/2009/02/sp0256-al2-creative-commons-sample-pack.html whose lengths (excluding the silence at the beginning and at end) are very close to the datasheet values. The guy who made the recordings got the chip to play all of the sounds, and then hechopped them up automatically using thresholds. For each sounds, he got the IC to play silence after each one has finished. Are you sure that the length values in the datasheet are wrong?