We had this issue - We have Sprint and several different Treo models. You have to get the wav file to play as a Voice Memo on the Treo. Its one of those 'do it once and it will always work this way' setups. Not too tricky to do, and everyone that did this has it working now.
1. From the email screen, select and open the email with the wav attachment.
2.At the bottom of the screen, you'll see the wav attachment. With the stylus, tap the icon to the left of the message to bring up these options:
View
Select Viewer
Save to Card
3. Choose "Select viewer"
4. Choose "Voice memo" in the drop down box
5. Click "View" to play the message
6. The message will transfer and be stored on the Treo.
7. The Voice Memo player will open and you can tap the play button to listen to the message (make sure the Treo speaker is turned on)
8. The message will be stored on the Treo as a wav file; to delete, highlight the message, tap at the top on "Voice memo" and you'll get a drop down box that will allow you to choose to delete the message.
You should only need to set this up once. Next time you receive a wave file, tap the wav file icon and the "view" to play the message.
Hope that helps you!