I ordered a couple of RAM sound units fro Harbor Models in the USA - RAM don't do an online mail order service, so Harbor Models got the order.
Shown below are the two units I ordered, RAM78 "Riverboat Steam Whistle", and RAM48 "Steam Engine Sound".
After playing around with them a little this afternoon, I have to say that I'm VERY impressed. They both have a couple of features that I didn't know about when I ordered, so I was even happier after I'd read the instructions.... The Riverboat Steam Whistle and the Steam Engine Sound are both digitally recorded sounds taken from REAL ships, so the sound is unbelievable! The units will operate on 9-18 volts, and come with a small connector to plug straight onto a PP9 type battery - This can quickly be snipped off if you prefer to use a 12v supply or whatever. The higher the voltage the louder the sound output.
As can clearly be seen in the photos, the units come with small 2" speakers already attached, but these can be changed for any 4-8 Ohm speaker you may wish to use. I'll probably change mine to 4" speakers and use the 12 volt supply from my model to power everything.
The Riverboat Steam Whistle unit will play a constant "Loop" of sound as long as the micro-switch is pressed - The instructions give clear directions on how to modify the unit so it will only play ONCE if you prefer....
The Steam Engine Sound unit uses a small magnet and a sensor to detect the speed of the motor, every time the magnet passes over the sensor a "Chuff" sound is produced - Again the instructions give detailed and very clear instructions on modifying the unit so that the sound is produced on EVERY pass over the magnet, every SECOND pass over it, every THIRD pass over it, or every FOURTH pass over it - So it's very easy to fine tune it so it sounds realistic for your particular model.
These modifications involve a small amount of soldering and moving a few wires on the main circuit board, but nothing too involved, and I doubt it would be a problem for anyone that's used a soldering iron a few times.
In closing, all I can say is that I would wholeheartedly recommend these units to anyone that wants a realistic sound for a model - The range of RAM units is incredible, and any sounds they don't produce can be manufactured as a "one-off" if you require something out of the ordinary.
Regards
Eddy