I have now fitted sound to my MOW.
I decided to go with the Massoth eMotion S (with electric loco sound) because I know this product and because the Dietz alternative seems to have gone up in price to EUR 70 since earlier postings which meant the price difference was down to EUR 10 and IMO not worth the extra complexity.
I tried using the sound bus plug off the LGB decoder board but the SUSI plug provided with the Massoth did not fit. Also I had read another article in German which said that the sound bus was not enabled on the small decoder. So I decided to wire the sound decoder directly to the track power.
Opening up the MOW I decided the best way would be to use the contacts at the back of the decoder plug (grey plug in picture below).

I traced the wires back to the motor block to find which ones were the track power (brown and white or "
bru
n" and "
weis
s" in German):
I soldered some 16/2 wire to the back of the plug, and ran that through to the "bonnet" or "hood" where I was going to install the sound decoder and speaker.

The disadvantage of using track power directly for the sound decoder rather than using LGB or SUSI bus from the motor decoder is that a) you have to make sure the decoder addresses are the same (or I guess you could make a consist?) and b) to program the sound decoder you have to isolate it from the motor decoder, so it is not ideal. In my case I needed to change the default from 14 to 28 speed steps and parallel only so I could use f keys above f12.
I found I had to reduce the default volume setting (CV 200) from 32 to 16 otherwise the sound was distorted through the 8 ohm 2W Massoth speaker.
If I was doing this again I would probably remove the LGB decoder altogether (as suggested by an earlier "postee") and install an XLS as the cost difference between an S and an XLS is not that much and it would make programming much easier. Hey ho.