I found out that Windows XP mode in Windows 7 has Windows Media Player 9, so I've exported the Windows Media Player File Open type registry settings if you would like to try importing it into the registry.
To install, close out of Windows Media player, extract the attached zip file, double-click the reg file and open. Then confirm to import into the registry. Then check if the 'wav' file type appears in Windows Meida Player->File-Open.
Note: Don't apply this registry file if you've already upgraded to 11, as this may remove some additional file types Window Media Player 11 has.
wm9_registry.zip (683 Bytes)