I've never completed FFX so...bear with me if this is wrong...

He doesn't actually fight his dad when he (Jecht) is a person...yes???

So this is one of the times he fights Sin...yes???

Which time...I haven't played the game for a long time as well so...if all the battle themes are the same, then my bad...

Here's s ite with lots of FFX midi downloads...maybe it is on here...

http://www.ffshrine.org/ffx/ffx_midi.php