If you are using the OSMC skin, just bring up the OSD (OK button) and select the icon that looks like a speaker. You can select the audio track from the menu that pops up.
EDIT: Kodi will play the stream tagged as default in the MKV file. It depends on how the MKV was created as to what track is tagged default. It’s fairly easy and quick to change the default using MKVToolnix.