Yes, you need to have a 16 Ohm speaker load for the 5v PS2 boards. I test my speakers before installing them to ensure they have the correct resistance no matter what is printed on them. Many times I find a number 4 & 8 Ohm speakers are 3 and 7 Ohms respectively.
Vernon may disagree with me on this solution, in the past, I cheated on one of my 5V engines and used an 8 Ohm speaker but added an 8 Ohm resistor inline to dummy the load up to 16 Ohms. This will make the speaker quieter but not by much. I ran this on an old RK engine that didn't have much value so if I blew the board... not a big deal. So far, 7 years and 225 hours of runtime later, it still runs & sounds like a champ.
