Chances are I can fix your PS-2 3V audio amp to restore sounds. PS-3 will not run a PS-2 Slave board. The PS-32 repair board will; which could get you the PS-3 sound file. That would be a better approach than going full PS-3 in the lead unit.
Audio amp fix would be much cheaper though.
Early PS-3 do use slave boards and require a tether. But the slave board is a PS-3 slave, not PS-2. It also incorporates a Tach reader, so it operates different than the PS-2 versions.
MTH did decide to do away with PS-3 Slaves and never made a PS-3 Slave kit like they did with PS-2. What you have is two PS-3 engines run as a MU or lashup. That does do away with tethers. I would also say, if upgrading a PS-1 Turbine going this route is necessary as it is a pain routing 10 pin harness through the 4 pin type the PS-1 used. Requires grinding on trucks and a custom length harness. G