Did you read the "Installation and Configuration Guide"?
http://docs.blackberry.com/en/admin/...1-5.0.1-US.pdf
Hell I'll point you right to it. Page 19:
BlackBerry Attachment Service
If the BlackBerry Attachment Service is hosted on a computer that uses Windows Server 2008, the BlackBerry Attachment Service does not support .mp3 audio files on BlackBerry devices, and the BlackBerry Attachment Service does not support any audio file formats on BlackBerry® 7100 Series devices that support CDMA networks.
You must host the BlackBerry Attachment Service on a computer that uses Windows Server 2003 if you want the BlackBerry Attachment Service to
support .mp3 audio files on BlackBerry devices and all audio formats on BlackBerry 7100 Series devices that support CDMA networks.