Let's talk SOUND!
It was my big OTW Type VII project where I first played around with the addition of sound to a working RC submarine. The module that I chose was a bit big, but was actually a lot of fun as it tied into the Electronic Speed Controll (ESC) of the model to offer proportional diesel engine sounds as the model was underway.
The speaker was mounted just below the deck grating, which actually worked out well in that as the boat dived, the speaker was submerged and the sound ceased to be heard. This could, given enough imagination, be interpreted as the crew shutting down the diesels and converting to electric power for diving (that's my story and I'm sticking to it!).
A luck would have it, I am now in the middle of yet another Type VII buildup, this time in a slightly smaller scale. Rather than diesel sounds, this model will feature a series of sounds that is controllable via the transmitter. The module that I elected to try was the Engel Modelbeau RSM Sound Module. At only $85 (at today's exchange rate), it's really not expensive. It has a small footprint and offers a lot of functionality.
The module is really two components married together, a receiver and an MP3 player. This marriage allows the playing of up to 20 different sounds, each one user-selectable from shore. Sounds are stored on a micro-SD card and can be changed out according to the desired sounds. The module comes pre-loaded with several sounds, some of which are applicable to my sub project. These include engine noise, sonar pings, radar, machine guns, deck guns and much more.
To play a sound, you simply move the control stick (or switch) in one direction and return it back to neutral. The number of the inputs is counted by the unit and it is this number that selects the sound. Holding the stick in one direction for about 1 second will activate the corresponding sound.
Other than engineering a waterproof connector to get the speaker leads out of your cylinder, the only other thing to worry about is a speaker. Engel offers a compact piezo speaker that is waterproof for about $20. I haven't tried that particular unit myself, instead electing to go with a larger diameter plastic speaker that I sourced online, but I'm sure it works well and it's recommended for use by them on their site.
To sum it all up, adding sound functionality to your model doesn't need to be as intimidating as it might sound. Just plug this module into your receiver, connect a speaker, and you're done!
Buy one, install it and play around. It could add a new dimension to your model and a lot of fun at the pond!