Most of the manufacturers just mentioned, also supply sound units for other ship’s noises, e.g. fog horns, air horns, bells, whistles, telegraph, sirens, destroyer 'whoops' etc. These tend to be the synthesised kind that is adjustable by the user as required. Some will be supplied with their own speaker and perhaps a microswitch so that they can be activated by a servo; others will need a separate speaker and some sort of r/c switch to operate them. You will need a spare radio channel for these sounds. You can operate more than one sound unit at a time, but if you are using just one speaker then you must fit a separate audio amplifier/mixer (e.g. ACTion P97 or P101), or fit a separate speaker for each unit. Two sound cards wired directly to the same speaker is almost the quickest way to a blown unit that I can think of and nearly as quick as connecting the power supply the wrong way around!
The question of speakers arises. The sound quality from a paper cone is without doubt better than any other type, BUT don’t forget you are only going to hear the sound from quite a distance, so real hi-fi quality would be wasted. Mylar or polypropylene cones are better for model boats because they are waterproof (or rather, splash-proof), in that they probably wouldn’t survive being submerged while turned on. Speakers do have polarised connections, i.e. there is a +ve and a –ve marked on the solder terminals. If your sound unit or amplifier is also marked with the polarity of the speaker outputs then do connect them up, like for like. However, it isn’t vital that they should be connected like that; you won’t damage anything if they aren’t.
shows a siren unit connected to an r/c switch with a flashing light unit wired in parallel. This is popular for fire and rescue models and shows how several of the different items in this article can be operated together. In this case the siren will sound and the light will flash when the switch is operated.