Mechanical reefer volume??

Don't know the exact number but it's huge.I had a temp job once stacking boxes of brussel sprouts in 57 ft PFE cars.  :> ) .....DaveB

On the N Scale version the

On the N Scale version the screw is on the bottom. There is a hole in the floor for adjustment. Don’t know about HO. GS

On bottom

Thank you, I found the volume control!  It is under the car.  There's a tiny tiny hole to slide a tiny tiny flathead screwdriver into so you can adjust the volume.  So sounds like it's the same for both HO & N scale for anyone having problems locating the adjustment knob.  

Confusion

EDIT: OP already found the solution, see above post...
(Posts crossed in transit ).

Dear ?? (Hanzlik),

You're not crazy, things are definitely "not as the diagram shows" here...

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Now, onto the reefer unit...

- I take the point that the instruction sheet you posted shows this

- A quick google shows this (admitedly small) image from a Nov 2013 MRR review of the model and sound PCB, which appears to match the manual/diagram 

- for educational purposes, the volume control on this board appears to be a "trimpot",
located as highlighted...

and looks something like one of these "in isolation"

(If you find one of these or similar on the board, even if it's not where the diagram says,
then you've likely found the volume pot...
...as GS notes, sometimes they are mounted on the back/underside of the PCB to be accessed thru a hole in the floor, maybe between the centre-sill members?) 

- Now, to the confusion, the PCB you show looks nothing-like the provided diagram.
(a lot narrower, and the component placement is obviously-different)

This makes me think that there may-well have been a change in sound module thru the production-run of the model, but no matching update of the provided diagram/documentation... :-(
(I take it you have the more-recent 2016-17 era production run?)

I would flip the car over, and look carefully for a 1/8" - 1/4" hole, likely located between the centresill members, with a trimpot "slot" or "X" pattern visible behind it... (as per GS's suggestion)

Happy Modelling,
Aim to Improve,
Prof Klyzlr

N Scale version

I also have a couple of these in N scale and the volume control is on the bottom. May just be me but it seems the sound is better when there are at least 2 of these running at the same time. I've run mine on both DC and DCC and so far, they have worked just fine.

How much did having sound in

How much did having sound in a mech reefer set you back?

More than my wife approved of

The two of them in N scale that I got back in 2015 set me back $90 + shipping from one of the big e-tailers. Was more than my wife approved of but I didn't have to send too much time in the dog house. If I remember right, the same two cars without sound would have been around $40 so I'd have to say it was about $25 extra per car.

There was a project in a back

There was a project in a back number MR to build a sound unit for a reefer - I never managed to get mine to work!

Now that I recall, I bought a

Now that I recall, I bought a couple of the HO Genesis reefers but without sound they were $36 each (MBK price).  I think the sound equipped were $65 each IIRC from MBK.