no longer hear sounds when sending mail

Do you have "Do Not Disturb" Selected?

Go to System Preferences - Notifications and see. If it's checked, uncheck it.

Otherwise - do you hear other system sounds?

Verify you are not muted.

Make changes to Mail>Preferences and test.

If no success you can Try resetting NVRAM/PRAM http://support.apple.com/kb/ht1379