Need a relay to trigger sounds ...

Tell us more about the 0.5V-18V DC trigger signal...especially on the low end.

Sounds like a comparator on the input to detect that signal might do the trick.  I doubt you'll purchase this off the shelf, but I may be wrong.

Hmm....don't know of such an animal (off-the-shelf).  I'm thinking GRJ's idea of a comparator is one way but that involves bread-boarding maybe 5-10 components and maybe $2.  I don't think you need a relay since the comparator circuit would be powered by the wall-wart and simply drive a transistor switch to apply that 9.5VDC to the sound module at the right time.  But maybe someone else knows of an off-the-shelf solution.

Can you rig up some kind of sensor to the Bachmann transformer that would trip (or release) whenever the knob/lever goes above the "zero" position?  There are some mighty small micro-switches, magnetic reed switches, etc.. that might be hidden with some ingenuity.  That switch could then pass the 9.5VDC current to a relay or directly to the sound module.

Or for the same effort as bread-boarding a circuit, perhaps you can open up the Bachmann and with our meddling and some luck find an electrical signal of sufficient voltage (to directly drive a relay) that turns on whenever the knob is off the zero position.

What about using the DC current that goes the table motor  to power  a dc  (probably a 12 volt will do) Use the contacts on the relay to power the sound module with it's own power supply

If I understand Gregg's idea, then if you can live with a voltage drop to the turntable it might be a fairly easy assembly job.  I'd go with as small a relay coil voltage that is practical/economical which I'd think is 3V.  I'd use, say, 4 diodes to limit the relay coil voltage to just under 3V no matter what the turntable voltage is.

In other words, the 3V relay coil will take away about 3V of your available turntable voltage which would limit your top turntable speed to, say, 15V if your Bachmann puts out up to 18V.  If your turntable goes backwards - probably with a DPDT switch to reverse the polarity to the motor - that switch needs to be right at the turntable (shown with green box) so that the diodes see the correct polarity as shown (with the little bands at the end of the diodes).

If you can live with the 3V "tax" on your voltage, then we can identify specific components but I'd say $2 for the relay, $1 for the diodes.  It would be useful to know about how much current the turntable uses.

OTOH since Gregg mentions 12V, perhaps he's referring to yet another method as I'd think you want as small a voltage "tax" as possible if co-opting some of the Bachmann output.

If you just put another string of four diodes in the opposite direction, you don't have to worry about the polarity of the motor drive, and no switches are involved.

About 3.5V.  A 3V relay needs about 3V to switch (actually a bit less but ignore that for now) so the diodes+relay steals 3V from the Bachmann output leaving 0.5V for the turntable.  When the Bachmann puts out 18V, the diodes+relay still steal 3V and the turntable gets 15V.  This is why I suggest a lower voltage relay than 12V.  You can probably find a 1.5V relay but they can be harder to find than 3V ones.  In any event do you have any info on how much current the turntable draws?  This diode+relay method requires a certain relationship between turntable current and relay current to operated properly.

As mentioned Gregg might be referring to yet another method.

I'll get under there today and put a voltmeter on the connections when I run the turntable.  Like to have it trigger the sounds when the turntable starts and cease when it stops.

Seems to me that 1.5 volts is very low to power the turntable motor, Who's table are you using?  Will the table actually move with 1.5 volts or can you dial up the voltage. I don't have any other ideas   

I have had a couple of these sounds units years ago. You may need a momentary  shot of current to trigger this thing But isn't there away to have it go continuously depending on the connection....  I had one installed in a box car... flange sounds and  clickey clack. I triggered it with an old post war whistle relay (conventional days)

If you can find a relay that will energize when dc current goes to the turntable motor , use the contacts on the relay to power (what ever it needs) and turn on and loop  ITTP sound unit.  I wish we could see the schematic a little better,, you may have to jumper the momentary option. I know I had one hooked up my MTH crossing gates with bell sounds using  a relay.

Center  top schematic, ??

Yes, the center top diagram seemed the most direct.  Basically want to replace the mechanical switch with an electronic one turned on and off by the tickle of the current from the Bachmann transformer used to drive the turntable motor.

Is this an untapped market niche?  Hundreds of turntables out there without sound. 

How slick to have a setup which would ramp up the revs of the sounds as the turntable started to turn, and vice-verse when slowing down and stopping the turntable.

I'd have purchased one for sure ...  

Heck, I can't find just the switch part; have the module, the speaker, the power supply, etc.