Yes I have reaper istalled on my system, I have imported the midifile, but what seems to sound ok on my system never does on theirs...

have you tried sending them a Reaper file that you have checked instead of the midi file that they import?
 
You may have to go to Cockos' forum and start a thread and ask there.  Maybe there is some default they have set you don't or visa versa.

Since it works fine on yours it may just be a matter of plugins.

This sounds like a sample/bit rate mismatch, mentioned by Mr/s. Edwards (dubdisciple), to me.
 
Kind regards,
 
Living Room Rocker

Living Room Rocker
This sounds like a sample/bit rate mismatch, mentioned by Mr/s. Edwards (dubdisciple), to me.
 
Kind regards,
 
Living Room Rocker

MIDI can have sample/bit rate mismatch?
 
Well that's the derndest thang Ah evah herd.

I have used MIDI files from Sonar and Pro Audio with Logic and Cubase. Though I have Reaper I have never done much with it. (I own a license) But I suspect that it could be any one of a number of things. First is the tempo the same? Then as mentioned is the sample rate the same? Also Sonar can adjust its tick resolution I don't know what that is with Reaper or if it can be adjusted. That is something I would look at. Plus look at the MIDI in Reaper's PRV and see if it looks right. 
 
Now as I started I have used MIDI files between those named DAWs and have never had a problem in one or the other sounding any different from Sonar or for that matter Pro Audio.
 
BTW as an aside, of all the DAWs that were mentioned above only Sonar can load a MIDI file as a native file format. One does not have to import it in Sonar. This alone can have interesting results if it is imported into a project with its parameters way off.
 
The short answer, to my knowledge it isn't a Sonar problem. Cakewalk the grandfather to Sonar was a MIDI sequencer.  
 
 

"Cakewalk the grandfather to Sonar was a MIDI sequencer."
 
Maybe for the sequencer function, and I find that Sonar in general is very good for working with ! But, damn, it is not very reliable when starting working with vsti and midi environment !!!! By cons, no problem when recording audio and managing audiotracks.

I just did a song using all soft synths. I have been doing this for awhile now. In the past I relied on hardware synths. I can't tell a difference with VSTis vs hardware as far as stability is concerned. If a VSTi is causing instability its most likely not Sonar's fault.
 
 

I don't think I've completed a project without VSTi's since, well, geez... I'm not even sure how many years!  CW synths, Komplete, XLN, Sonivox, loads of freebies, etc., they all work well here!
 

No issues with VSTi's here. The reason I use Sonar is because it is *the* 'swiss army knife' of midi editors. Imagine you decide to edit a midi produced on an an arranger and decide to run it through Sonar - using Garritan libraries. You're going to need to convert (for some instruments) all the modulation control codes into aftertouch codes if you want it to both have the correct volume, as well as correct tremolo articulation. In Sonar, you can do this with just a couple of clicks.