The amplifier chip in all of the JJC units which I have handled is a Philips 15W job (@18v). It's the same one that we fit to the new P101, and also the same one as was fitted to the RCMW units. At 12v this puts out about 7W which is way too much for most 2 1/2" speakers. Richard is nearer the mark with a 4" x 8 Ohm one, but it should be 10W rated. See John Elsy's posting about the volume he gets with his RTTL with the same amplifier chip and a 4" speaker. One of the main problems with JJC engine sounds was the lack of adequate heat-sinking on the amplifier - not that important for momentary sounds like horns but not good for those which run constantly. Turn up the wick fully and the thing is going to overheat and die at some stage.
If you arrange to enclose the back of the speaker in a box, so that the air pumped out from the back of the cone never interferes with the air coming off the front, then you'll find a significant increase in volume. Mount the speaker and box firmly on a (baffle) board affixed inside the hull, to increase the resonance from the hull itself. A sound tube can also help by keeping the column of air from dissipating inside the hull, and you must - of course - allow a reasonable opening for the air (sound) to escape. No point in having just a small grating or a half-open vent - that's like turning on the radio, then leaving the room and closing the door behind you.
FLJ