To prevent the two power supplies from interfering with each other, I added a common anode dual diode (STPS3045CW). Here is a datasheet from the manufacturer. I salvaged the diode from a switch mode power supply.
The green LED indicates power from the bench power supply. If it is on it will be the dominant power source. The orange light indicates the alarm power source. If both power supplies are on, the bench power supply will do the work.
The blue part is the amplifier. I use it all the time when I am listening to music. My tablet by itself has tiny speakers and therefore no bass whatsoever. I bought the amplifier through amazon. http://amzn.to/2HCxCOw