This piece rose from a single vocal recording of my girlfriend's soul singing and a recording of her heartbeat. Every sound you hear has been hand-crafted from these recordings in Steinberg HALion 6, no other synths or samples involved. Production was done in Cubase with Waves plugins for mixing and mastering.
I had 2 approaches to sound design on this project.
One was more of the creative/curious/random kind, where I utilized snippets from the original recording in the sampler or as a wavetable source. The "ah" choir pad (e.g. at 3:30) is a sample based instrument, the "oh-oh-yah-eh-ih-ye" pad (coming in at about 1:13 and 3:37) is a wavetable sound.
The other approach was more technical. I scanned through the recordings looking for "clean" or "pure" sine waves, saved them as wav files, ordered them by "cleanness" and created a wavetable out of them. For the other basic waveforms I used GNU Octave to create saw, square and triangle waveforms from the sine wave samples I saved earlier and also made wavetables from them. I scan randomly, but slowly through the wavetable to create a permanently changing sound source.
To create noise I collected everything from the recording that could be used as such, like breaths, f-, s- and sh-sounds and the like. I lined them all up on an audio track, added delay and reverb to diffuse the sound and make it more noisy. I layered a copy of the track on top of it, but shuffled the order of the samples and changed the reverb and delay parameters a bit. I repeated that process until I found the noise was dense enough. Finally I used an EQ to make it sound like white noise. I bounced the results to disk and got a roughly 45 seconds long noise sample, which I used in a sampler as noise source. The sample starts at a random position every time a note is triggered.
After that, the rest is essentially subtractive synthesis, modulation, layering and postprocessing.