I just removed a small amount of material from the back of the coupler and box so the truck would clear.   

I use a pin vise with a drill just a few ths smaller than the nail. Drill the hole deeper than the nail, then use a nail set to push in the nail just above the tie. After your satisfied with the track you can ballast and glue.  When dry you can remove the nails very easy.   If you select the correct drill size the nails will go in fairly easy but still hold.   

If the coupler is not split, you can use Locitite retaining compound (620 or 680) to secure it to the shaft. This is not a super glue, clean up both parts and rough up the shaft and apply a very small amount in the coupler being very careful not to get any in the bearing. Let it setup for 10-15 minutes and it will not come loose again.

The sound board with speakers just plug in, it would be very easy to remove the board and make a jumper plug to turn the loco into a standard DC only loco. You don't loose any money you still own the DDC/sound board, which you could sell.