Generally, there is no good reason to split the return plane. At frequencies higher than 100 kHz, the return path runs directly under the signal trace, so you can use the concept of partitioning high speed and low speed (or audio) circuits away from each other. Refer to the PC board design for low EMI link I provided previously for more info on this.
I always advise ensuring a solid and adjacent return plane for high speed signals (>100 kHz). Lower frequency or other non-critical signals may be returned on the power plane, BUT I always recommend all signal layers have a solid and adjacent return plane, especially for dense or mixed signal designs.