Small and simple is not the normal LGB mogul sound unit!!
Original was 2 boards stacked in the tender.
Next was a board with super caps (no battery needed) in the tender.
Possibly the 6500x series, but this did not have a 9 volt battery connector.
So, I am not sure what you found, a picture would be very helpful here.
There was a toytrain tender that had what you described for a board, but it had random sounds.
That’s the unit LGB used in their sound boxcar. Way back when.
As John commented this is the basic analogue ‘steam’ sound with chuff only. This board is very, very old as no volume adjust potentiometer.
The connection is very basic with two track (power) leads and an optional battery connection. The battery is preferred for better low track voltage operation. Chuff is voltage dependent and not really ideal for a Mogul but I have in the past used these boards for similar sized locomotives.
LGB 4268S was the car produced in 1992-1996 with that sound unit.
4267S was thew diesel sound car.
The original board installation the power coming from the donor locomotive (remember no track power pickups on the original sound car the board came from) were connected to the red and black wires with the yellow insulating tape attached to the end. If using track power pickups then attach track power to these two wires. Alternatively, power could be attached to the yellow screw block terminals, but if using the red and black wires as well then ensure correct polarity.
Search for this manual by typing in google:
LGB 4x352_072002.pdf
Also, MTS sound version:
LGB 4xxx2_052004.pdf
Very useful for changing sound function assignments in DCC.