By default, MediaMonkey uses a set of rules to guess any track
properties that are not defined in tags. If MediaMonkey is regularly
guessing track properties incorrectly, those rules can be easily
changed.

These rules are stored within the [SongMasks] section of the MediaMonkey .ini file. 
To change the rules used to guess track properties:
Close MediaMonkey (so that it doesn't overwrite any of the changes)
Back up the .ini file. If you're not sure where to find it, see:
 http://www.mediamonkey.com/support/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=17&nav=0,2

Open the .ini file inside a text editor such as notepad
Edit, re-order, or delete the masks in the [SongMasks] section of the file.
The variables are not documented anywhere, however, it's fairly easy to
figure out what they are by comparing masks in the .ini file to those
which appear in MediaMonkey. Some of the basic ones are:
%A=Artist

%L=Album

%T=Track#

%S=Title
Save the .ini file.
Restart MediaMonkey