I have never added sound to any of my boats, because i didnt see a need, since all i run, except for two cheap warships i picked up, are fast Catamarans, but, guys i know who have added sound usually get the sound controllers from these guys:
https://www.harbormodels.com/sounds.html I never really paid much attention to it, but, from what i understand, depending on the type of sound, and if the sound controller will also do engine startup sounds or not, it either hooks up to an extra channel on the receiver, or between the ESC and receiver. I envy you for being able to finish a wood kit. Years ago, i tried to build a wooden sail boat, and i didnt do it very well, since it sank twice on me in less than 2 minutes, so i gave up and went back to prebuilt race boats.