When my computer is sleeping it still plays the 'new mail' notification sound. Is there a way to disable this? It's quite annoying to hear it all night long.
I have my energy saving preferences to put hard disk to sleep after 3 hours.
To start with, your Mac is not asleep. the Mail.app is notorious for preventing sleep. However most sleep issues can be fixed by clearing the PRAM.
To clear the PRAM, power off the Mac - Then power it back on while holding Command+Option+P+R. Continue holding the keys until the Mac reboots itself again. With any luck this will allow your mac to sleep normally again.