Try using the DK (cash drawer kick) output. Check with the Epson Technical Guide. One pin is at 24volts (pin4) When the printer receives the appropriate control command then the 24v is applied across pin4 and either pin5 or pin2. You can connect a 24volt buzzer (available from most electronic suppliers for a few Euros). Setting the pulse duration to 500ms should give a good buzz and of course you could repeat the command a number of times.