You may find these directions helpful:
http://www.cisco.com/en/US/docs/voice_ip_comm/unity/white/paper/cuaudiocodecs.html#wp64774
To Install the G.729a Codec for Cisco Unity on a Subscriber Workstation That Does Not Have ViewMail for Outlook or DUC for Cisco Installed
Step 1 On Cisco Unity DVD 1 or CD 1, browse to the Utilities directory.
Step 2 Copy the Sl_G729a_setup.exe file to the directory of your choice on the network or to a disk. Administrators or subscribers can then install the codec from the network or from the disk:
a. Confirm that the subscriber workstation is using a Windows operating system and that a compatible audio player is installed.
b. Double-click Sl_G729a_setup.exe, and follow the on-screen prompts.
c. Restart the subscriber workstation for the codec change to take effect.
This should allow Windows Media Player to play the codec.
Thanks,
Scott