There is a particular term for the mispronunciation of a word based on
the way the word is spelled as opposed to the way the word should be
pronounced. For example "off ten" instead of off in" What is that
term?

shimel -- 

Are you thinking of "phonetic," a linguistic term that is commonly (if
not technically correctly) used to refer to the pronunciation of a
written word "the way it sounds"?  In your example, "off-ten" is the
phonetic pronunciation of the basic sounds ("phonemes") in English of
the letter combination "o-f-t-e-n."  The word does not always relate
to mispronunciations, but it "sounds like" it may be the word you are
looking for.

markj-ga

Surprisingly, "off-ten" isn't a mispronunciation, at least in historical terms:

"Topic: Often" (Word for the Wise, Archive, February 18, 2004 Broadcast)
Merriam-Webster OnLine
http://www.merriam-webster.com/cgi-bin/wftwarch.pl?021804

I am not certain if it relates to a phonetic term.  I know I heard it
used on a NPR program and in fact the word they used as an example was
"often".  The comment after that was, " one would not say lis ten as
opposed to listen with the t being silent." So in the sense that the
term has to do with pronunciation, I would imagine it could very well
be a phonetic term.

for Markj-ga

See comment I added below.