Background: my email is delivered to my OSX machine via sendmail. My ISP sends mail every time I make a connection. I leave Mail.app running 24x7 and have set it to scan my Unix mailbox every minute.
There's a bug in OSX 10.0.* and 10.1's Mail.app that prevents audio notification for unix mailbox deliveries. A simple solution for this is to use Mail.app's mail filtering rules. Enter Mail.app's preferences, then ...
select "Rules"
select "Create Rule"
select Criteria "To" + "Contains"
type "@" in the text field.
Enable "Action "Play Sound"
Choose a sound to play when mail arrives.
Click 'OK'
Now drag you new rule to the top of the list. Note: I used "To:" "Contains" "@", so that any mail with an "@" in the recipients address triggers the audio notification. Local mail, e.g. cron generated output, is sent without "@" in the address, so I don't hear their arrival.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011022071007903