Lion email loses sound

I have an iMac with Lion OSX 10.7.3. The email app keeps losing sound notifications such as the "Whoosh" when sending mail and the "boop" after polling mail servers. However, sounds often returns for no apparent reason. It seems that many people have a similar problem and most advice is to turn of the "idle" command for polling IMAP mail servers. However, all the mail servers I use are POP3 and there is no idle command with these. Other advice is to ensure that the "play sounds for other mail actions" box is checked, which it is in my email preferences. Can anyone suggest a reason for this inconsistent behaviour?