Something like :
fetchmail -ks && beep
fetchmail exit status is 0 if there is new mail and non zero if no new mail or error.
I don't know if you want your fetchmail to actually *fetch* the mail. But if don't want to, use 'fetchmail -cs' instead of '-k' so that it only check if there is new mail.
beep can be the one of the beep command (package beep on debian) with customized beeps.
Hope this helps.