[Lingo] kanji pronounciation question

[s smith <sjs@example.com>]

Hi all.

There seems to be a pattern on how a kanji is pronounced when in the 
first position of a word or a following position, but I can't quite put 
my finger on it.  An example would be 説 as used in 円説 or 演説.  円説 
is a short phrase and might mean money theory.  The second is a single 
word for a speech.  説 in the phrase is pronounced せつ but in the word 
is ぜつ.  Is there a rule or pattern that will help me predict 
pronounciation of a kanji here?

Complicating matters in this case for me is that せつ・ぜつ isn't in the 
音 or 訓 for 説.

Thanks
Steve S.