[exim] "Beep" on incoming mail ?

Hello all,
I have successfully installed exim 4.50, I have a small Linux
distribution that serves as a router called FREESCO, see
www.freesco.org.
Now I would like to make it beep whenever it receives a message, the
command is simple : "[root@freesco] beep" makes it beep.
How is it possible ? I have tried to put the command in the exim
system_filter file but nothing happens...I'm kind of lost.
thanks a lot in advance,
best regards,
M. CATTIN