Hi Sijmen!
I'd like to take a few moments to give you some feedback on your arrangement of In Dreams. I'll do my best to answer each of your questions below. Since I have no idea what your musical background is- if I seem like I'm talking down to you- I'm not. Only try to make sure what I'm saying is easily understood. :) So here goes....
1. It doesn't sound like a real orchestra. How to improve that?
Answer: The orchestral samples in Garageband are not superior quality- so you'll never get that super rich sound of the high end samplers or of a live recording. There are several things you can do to tweak the sound though.
Panning- I'd work on balancing out the tracks and making sure not everything is coming out from the center. Imagine a live orchestra and think about where the sections sit. Try and emulate that with your panning.
Effects- when the flute is playing in the beginning it is very dry. If your version of Garageband has reverb, try adding some. This will soften up the sound and make it sound less artificial.
Arrangement- even with excellent samples, if the arrangement is not condusive to how an orchestra, it can ruin the impression. For example, it is very rare for everything thing in the orchestral to be playing unison (all playing the same thing at the same time).
Mix: make sure you are paying attention to your tracks volumes and setting them accordingly. Change of dynamics are very effective as well.
2. Would there maybe be a pre-written orchestral arrangement available somewhere with the different parts written out? I've only got two hands of piano now and the rest is improvising.
Answer: Try googling and you'll be suprised what you find. I thought the strings coming in with the piano and flute sounded nice. I'd stay away from the staccato chords in the piano though (short chords) because it contradicts the legato (smooth) line of the flute too much. If you are wanting to sustain the piano chords in the beginning, that would work better.
3. Are there any freely availably software instruments for GarageBand? Of not, for Logic Express? Is the latter worth the investment?
Answer: I'm afraid I'm a Sonar and East West guy. But I can assure you they are worth every cent. For example- take a listen this orchestral piece I wrote with those two programs. I couldn't do that with lower quality samples.
Orchestral SampleSounds like you're making good progress with the piano lessons! Keep it up!
I hope that helps you out some! I love working with music and computers, I find it very inspiring and hope you do as well!
Thanks for sharing!