The simplest solution may be to create a special version of the music with the quasi-repeat copied out and edited just to create the MP3.

The other solution is to create an expression to switch the instrument MIDI channel to an empty (unassigned) slot in the Aria player for the repeat pass you do not want to hear the player and then create another expression to switch the channel assignment back for when you want to hear the player. The same empty/unassigned Aria slot can be used for all instruments that are silent on a given pass, but the expression to switch back needs to be tailored to each instruments original MIDI channel.

This works better than changing the velocity or mod-wheel settings since some instruments use one for volume and others use the other.

I have been trying to use, without luck, 2nd time only playback or play first x only, or  and it does not work. your solution is too complicated and time consuming. Ideally, one has to focus on the flow of arranging, writing, creativity and trying ideas and not on being a computer or midi geek.

Off course I can add the section without the repeat and once I have the complete arrangement then deal with adding the repeats, this is another step, again time consuming. Finale, please add Xst time only to playback on repeats. how hard that can be?

I routinely use dynamics such as mf 1x only or f 2x only or mp - mf (under Windows). However as implemented, these controls set one dynamic level.  You also have to set the other dynamic level to define the change with either a hidden dynamic or one with edited text to a blank string.  For example, for f 2x only I would in the bar before the repeat set pppp (and change it to zero velocity).  Likewise mf 1x would require pppp (or equivalent) prior to the repeat.  You'll find numerous examples at http://bandmusic.co.uk/brass.html 

A further complication is that Finale cannot process different dynamic levels for different layers on the same staff unless they are linked to different MIDI channels (which it's sometimes awkward about).  In my experience different dynamics on different staves for the same instrument and MIDI channel usually works but the safe option is to assume it doesnt and set different MIDI channels.  (Different MIDI channels are automatically used for different instruments).

You may encounter a further problem with busy scores: Finale cannot process all the notes fast enough and will omit some notes and/or changes in dynamics etc towards the bottom of the score.