To work with this file, you will need:-
(1) LMMS for Mac (please download version 1.1.3, from https://lmms.io/download/#mac
(2) A free (MIT-licence) soundfont file called FluidR3_GM.sf2 (you can find it using your favourite search engine)
Then install the soundfont file into LMMS by opening LMMS and selecting edit --> settings --> paths --> default soundfont file and browsing to it.
(3) Then you will need my LMMS version of your music, which you can download from http://www.mediafire.com/file/vy24vd52on8caw7/Widelands2.mmpz
If you open that file in LMMS then you should get a screen that looks roughly like this:-
(It will perhaps look a bit different because I'm using Linux)
To change one of the choirs into a violin, you can click on it which brings up a menu:-
If you click where it says "Patch", then you get a second menu which is all the instruments you can choose from:-
Optionally you can change the name of that music track to something like "Violin #2"
If you wanted to make the bass deeper, then you might click on the "Upright bass" tab where there is a little cog wheel on the left, and select "clone this track". Change the cloned track to a different instrument (I suggest tuba).
You can obviously add more music tracks with other instruments as well. Play with it, it's fun!
All the best