DCE clock rate and Voice Quality

My question is about VoIP voice quality. When I configure a clock rate (28800 or lower) on a DCE interface on the router and using a G.729r8 codec that has a default requirement of 8kbps for one call, I cannot seem to hear anything from the other person but configuring the serial interface (DCE) with a higher clockrate, the voice quality gets better. Can someone explain why this is the case. My network scenario is in a lab environment.