Finally I found the strings version of the fantastic piece "Voyage" (http://www.youtube.com/watch?v=u-PE7c24G0I) (My personal favorite of all the music but this version is not on any OST or single).

It's available here (http://itunes.apple.com/jp/album/id208196821) (alongside the piano version)

However, I don't understand Japanese and I don't use Itunes. So if anyone knows where I can get it elsewhere or would be even so kind to share it, I would appreciate it.

Download Fantastic Children Music Collection torrent - BakaBT (http://www.bakabt.com/128061-fantastic-children-music-collection.html)

The version of Voyage I seek is not on these OSTs. It is used many times in the series but just strings & flutes. I can only find it on the japanese iTunes site.