I've played both games all the way through (I own them) and I also own both of the soundtracks, but I wouldn't say 'horror' is the best way to describe the music. And, no offense intended, but perhaps if you got the real track names. For example, Track 1 is called Icaro.
Anyway, although this is tempting, I'm in the middle of making my own Shadow Hearts/Shadow Hearts: Covenant AMV, based around the story of the game, and in my opinion, making an AMV using solely the OST music from the first game will, well, leave it fairly empty. If you're going to do it, I suggest you try and splice some of the songs from the first game (such as Sign of Him [The Creation of God] with Evil Gate Opener from the second game.) Also, use the chant from Icaro only sparingly and where it sounds right, otherwise it will literally sound horrible. Try to mix in some of 'The Fate' as well, and of course, 'Sacrifice - Alice'.
Further, you may want to adjust your plan with the music. Mixing together 8 different songs won't work to create the imagry you want to see. Stick to one song to be your base, and edit that one. Unfortunatly, the OST for the first game probably isn't the best choice, as you pointed out, it's mostly sounds odd chords, and short melodies, with only the ending track and the bonus track at the end (track 36 'True Voice') on the second disc. I would suggest perhaps you take a look at the soundtrack for the second game, which features several songs to suit your purpose, such as 'Icaro Again,' 'The Fate,' 'Evil Gate Opener (parts 1, 2, 3 which can be merged to make one continuous song), and the ending track (I don't have the spelling memorized in Japanese, but it means Love Flower in english).
I have done my own editing, splicing, and merging of songs from these soundtracks, and I know what I'm talking about, so don't be stubborn and just dismiss this post. Of course, you're free to do as you like, but hopefully this insight will help you come out with something close to what you're looking for.
As the warlock said in the end, even if another age of storms buffets the world, I intend to go on living, and fighting, for you who saved me from the darkness, the one I love!
New Work: The Second Call
| A Cavernous Soul