OK, thank you.
Under Options > Tools, I created a tool called Launch Player, pointed it at MediaMonkey.exe, and checked the box to launch this tool on double clicks.
Now, when I double click on a track, it opens an instance of MediaMonkey, but it does not play the track. If I now double click on another track, it opens a second instance of MediaMonkey, but does not play a track, either.
What I need is a way to play a track in MediaMonkey (my default player) when I double click it. I can't figure out how to make this happen.
I have removed iTunes from my drive, because Mp3tag would open tracks in iTunes when I double clicked, even though MediaMonkey is set up as my default player. (And iTunes takes forever to open, and it is a resource hog.) After I removed iTunes, double clicking a track simply returned a message "Cannot open the file ...... for playing." So then I tried creating the tool to launch MediaMonkey, but I obviously did not know how to tell the tool to actually play the track, and to not launch a new instance of MediaMonkey on each double click.
Still trying to figure out how to double click a track and have it play in MediaMonkey (or anything other than iTunes).