The advent of DCC has simplilfied the addition of on board sound. As a result, sound has rapidly increased in popularity. It seems as if your layout cannot be considered state of the art without trains literally roaring down the rails. A sound equiped locomotive is an attention getter without a doubt. But locomotives with sound decoders do not come cheap. So is sound just an expensive novelty or a significant contributor to our experience of a layout? It appears to me there are at least two questions to be answered before I invest in sound.
First, I find sound in N scale locomoitves vey uninspiring. It does not seem possible to generate sounds with much depth from the tiny speakers which can be fitted inside a N scale shell. Larger scales are at an advantage with more interior space for larger speakers. But resonant base sounds are still missing even in HO locomotives. Sound decoders typically add $100 to the price of a locomotive. I anticipate using up to 30 locomotives on a medium size layout. Even if only half of the locos are equiped sound, that would be a sizeable investment.
Second, this summer while visiting the Rogue Valley Model Railroad club I had my first experience with a layout on which several sound equipped locomotives were running. With one or two locomotives running around the layout and others parked on sidings or in yards the sounds of the locomotives seemed to me to be something of a distraction, even annoying. This was a large club layout and there were no more than a half dozen locomotives emitting sounds. What might it be like in the smaller spaces of home basement layouts?
The Soundtraxx Multi-train sound system attempts to solve the issue of too small speakers by dividing a layout into zones each with its' own larger speaker. The system routes the appropriate sound to each speaker as the locomotive is detected in that zone. You do experience a fuller sound. However, the system is limited to layouts with Digitrax DCC and to one train at a time. While sound does follow the train around a layout, people do resport the sound is not sufficently locatized so it is easy to detect sounds are not coming from the train itself. And the Soundtraxx device may not be as costly as equiping an entire fleet, it is not cheap.
Back in April 2012 Lance Mindheim demonstrated another answer in his blog. He connected wireless high quality headphones to a stationary sound decoder which is consisted with the locomotive being run on his layout. He gets the rich sounds akin to what is experienced in the cab of the locomotive. This system works well enough for a lone wolf operator. But I see problems for any operations involving multiple trains. How do you communicate with a dispatcher or other operators? In the multiple train scenario there must be communications with other persons, something made difficult when wearing noise cancelling headphones. And it does not allow for visitors to participate in the experience. Lance's response is layout owners most frequently operate by themselves anyway. He also suggests in his June 27, 2013 blog entry, the possiblility adding a sound mixer to the system as a way to over come the isolation.
I am not into the X-box, PlayStation, nor the on-line gaming culture, but it appears to me their virtual worlds may offer some solutions for model railroading. Game players have a rich sound experience and can also communicate with other players. Headsets used by gamers have a boom mike to provide communication with other players. Some headsets are stereo with two earpads, but others are monophonic and cover only one ear. This sort of headset might not provide the totally immersing sound Lance cherishes, but it would allow awareness of the larger surroundings. Bluetooth technology is widely used for wireless digital communications. While bluetooth has a limited range of about 30 feet, a centrally located transceiver would be adequate for most home layouts. The complexities of this multi-channel route exceeds my knowledge of the field, but it would seem worth pursing. Randall Morris has been working on a project which he calls Virtual Sound being incorporated into the JMRI software.
While headsets can provide something like the experience of being on board the train, it does not replicate the railfan at trackside experience of passing trains. Perhaps some combination of something like the Soundtraxx system and the current on board sound decoder is as close to that experience that present technology can provide. The best solution for sound on the layout might well be a combination of all of these approaches.