You can still use the method listed here even for Outlook 2007;
http://www.howto-outlook.com/howto/addingsoundworkaround.htm
Whether it will play or not completely depends on the receiving mail client.
Outlook 2007 for instance does no longer support this (even though it can
send it). Also, many (if not all) web based mail clients will not support
this either. If it is really important to hear the sound while reading the
message, publishing it to a (password protected) section on your website
might be a better way to go. Then you only have to send the link to that
person to collect the message. You can also attach the sound clip instead
and tell them to start the sound manually.
