For BES:
Upgrade to BES 4.1 SP2
The WAV attachment distiller is enabled by default.
Possible alternative?:
BES 4.0 SP6
Enable the WAV attachment distiller
Open the BlackBerry Server Configuration panel (Start > Programs > BlackBerry Enterprise Server > BlackBerry Server Configuration).
Select the Attachment Server tab.
Change Configuration Option to Attachment Server.
Ensure that Audio Attachments is selected.
Change Configuration Option to Connector Configuration.
At the end of Format Extensions, type wav.
Restart the BlackBerry Enterprise Server.
Above info taken from KB05446:
BlackBerry KB05446
For BIS:
Use Virtual View's AudioPOP - $14.99
Wait until your carrier upgrades to BIS 2.3