Create a script called event.addline
Post the following code in that script.
java.awt.Toolkit.getDefaultToolkit().beep();
Find ticket.buttons
Post the following in ticket.buttons
<event key="ticket.addline" code="script.addline"/>
Restart.
In windows you can go into control panel, then sound and you can change the default system beep to any sound you like to customize this feature. I like it so the customer can hear as you scan items to the cart.