Sierra Sound Systems is well known for their outstanding steam sounds, especially the D&RGW series of locomotives.  They have recently released a series of diesel sound systems under their SOUNDTRAXX brand name.
       I chose their system for ALCO diesels with the Leslie S3 Air horn to install in an Aristo FA.  This system does not come with a speaker since Aristo is installing speakers in all their diesels starting this year.
       My diesel is an older unit, so I had to order a 2.5 inch speaker with the sound unit.  I also ordered trigger reed switches and a remote volume switch.
       The system comes with a 31 page manual which is well written, and covers all parts and features for the system and its installation.  The Sierra system has diesel sound, bell, horn and dynamic brakes.  There are options available for coupler clank, and a variety of lights, including MARS light, Pyle-National Gyralite, Dual Oscillating headlights, Western Cullen D-312 Rotary Beacon, Prime Stratolite, Xenon strobe, and flashing ditch lights.  I did not use any of these, since I am modeling an earlier era.
       The installation of this system must be laid out carefully.  You have to install the sound board, speaker, volume control switch, charging jack, battery, and on/off switch in locations that are convenient for access.  I chose to locate the speaker in the rear of the car body facing up.  In various chat rooms on the internet, most modelers recommend this location for this diesel.  Sierra recommends that you build a speaker enclosure from wood or styrene.  I used acrylic to build a speaker box.  The reason for this is that a speaker makes it's sound through air pressure.  When it operates, air pressure is pushed both forward and backward, and if there is no resistance to the rear, a lot of the sound to the front gets canceled out.  I had to rebuild my speaker box to make it deeper to enhance the quality of the sound.  The formula is to make it the same depth as the width of the speaker, which I did not do on the first installation.  I also substituted a 2.5 inch speaker ordered from Phoenix Sound which has a larger magnet than the speaker that comes from Sierra.
One other modification was to put a baffle inside the body shell to close off the speaker area from the rest of the locomotive.  This is to prevent sound from being lost in the shell, and to force it out of the louvers.  I also removed the fan and smoke system since I needed the room and don't use smoke in my diesels anyway.  The battery and sound board were mounted in the battery box under the diesel.  There are holes in the bottom of this box which should dissipate heat.  I used the round hole in the box to install a charging jack, and replaced the Sierra DPDT toggle switch with a slide switch from radio shack.  This switch fits nicely in the slot already cut into the battery box.  I mounted the remote volume switch I the frame under the loco so that it would not interfere with the swing of the trucks.
Since I wanted to control the horn with a reed switch and track magnets, I mounted the reed switch on the bottom of the battery box.  This gives a " clearance from the switch to the top of the rail.  Care must be taken so that the reed switch will clear crossovers, turnouts, and built up grade crossings.  A good source of track magnets for activating these reed switches is Radio Shack.  These magnets are just as strong as the LGB track magnets, but a lot cheaper.  This tip on magnets comes from Don Odle of the Tampa Bay Chapter.  I took the loco to the test rack to begin programming the system.
A word of caution here, read the directions several times so that you are sure of what you are doing.  Sierra's system has the most options of any sound system on the market.  The system comes factory programmed for voltage regulated control, but you can change each sound to be either voltage activated, reed switch activated, or DCC activated.
Programming is done through switches and LED's on the circuit board or via the remote sound switch.  You step through each program position to reach the option you want.  The horn blows once each time to push the switch to let you know you have reached a different feature.  The option I wanted to change was the horn control since I will be using track magnets to activate it I believe there are 20 programmable options with this system which should make any railroader very happy.  I also changed the voltage level that turns the bell on and off.  If you use Aristo PWC for track power, you need to set the throttle response to HIGH or your sound system will be running in "Notch-8" when the loco is barely getting up to speed.
On the test track I slowly turned up the power and got the startup sound with the typical ALCO clanky idle.  The horn blew two short blasts and the bell began to ring as the locomotive moved forward.  As you increase the power the bell cuts off and the engine revs all the way to notch-8 as the speed increases.  When you slow down you get the sound of dynamic braking.  Real ALCO diesels slowed down by actually cutting off the fuel supply to the prime mover, and this sound system gives you the same effect.  As you slow down, the bell begins to ring again until you stop.  At this point you have the idle sound and a single horn blast.
When you go to reverse, the sounds work the same way except you get three horn blasts.  You also have air compressor and pop-off sounds which add to the realism of the system.  When running, the horn sounds the proper grade crossing when the locomotive passes over the track magnets.  It will probably sound when going over Kadee uncoupling magnets, but that does not really matter to me at this time.
In all, this is an excellent sound system with relatively easy installation.  Technical support from the company is good and customer service is excellent.

select each option shown in the handbook.  This is not complicated, but may be intimidating for a first time installation. Read the handbook sevearl times before you get started.  Remember, you can always bring the system back to it's default settings if you are not happy with the results.