plain emails with WAV attachments do not get picked up as voice mail messages - there are several extended MAPI properties that need to be configured on the message before Unity will consider it a voice mail message.
If you're interested you can look at a small example of how to send a voice mail messages using CDO off my site to get an idea of how to go about this:
http://www.ciscounitytools.com/CodeSamples.htm
Also, if you want to do it the easy way, check out the new IP Fax Configuration Wizard - it's capable of routing mails with WAV attachments in as voice mail messages for you:
http://www.ciscounitytools.com/App_IPFaxConfigurationWizard.htm
