use voice class codec in global config listing codec preference and voice-class codec in your dial-peers. let me know if this works.

Say I decided to use 20 for my voice VLAN...Voice Server Port config (so that voice server is on it's own vlan):switchport access vlan 20Phones port config(so that phones know which vlan to join):switchport voice vlan 20Router port config for voice v...

This just determines what codec is used to play the WAV file. Does not change the storage format to MP3.