With all of the different parameters available to tweak, it can be difficult to set up something that sounds nice when you're done. Here are a few tips on setting up an instrument:
Turn down the gain. Every gain knob (i.e. an amplifier), this is a gain stage. With every gain stage you have, it's easy to overdrive your signal - which means the signal gets distorted by clipping. In addition, if you have two samples that, by themselves, peg your meters - what do you think happens when you combine them? That's right, you overdrive the signal again.
If things sound bad and distorted, start by turning down the gain setting on the layer... especially if it's larger than 1.0. Then turn down the instrument gain. Then any gain on a LADSPA effect. Then the fader on the Mixer. Then the master output fader.
Test samples at full velocity. Your sample will be played louder if the velocity is higher. So, if you set everything to sound nice and full with velocity at 0.7, what will happen when you get a full velocity of 1.0? (Hint: clipping.)
Try to use samples that are -6 dB max. Visually, this means samples that peak at only 1/2 of full scale. Otherwise, turn your layer gain to about .5.
Remove all DC offsets from the sample. In a sample editor, there is usually a line down the center of your sample's waveform. This is the zero-line. The beginning of your sample should be on this line. The end of your sample should also be on this line. However, if your signal is a little above or a little below this line, you will hear a click at the beginning and the end of your sample whenever it is played. If your sample editor doesn't provide any tools to fix a DC offset problem, you can eliminate the noise by putting a slight fade-in/out at the ends of your sample.
The ADSR will not be longer than your sample. If you have a short sample, it doesn't matter how long you set the attack and delay - the sample will stop playing at the end.
Things change with the sample rate. If you have a really nice setup with all your parameters painstakingly tweaked... things will change if you change the sample rate of your audio card. Many of Hydrogen's internal settings and parameters are based on how many samples go by, not on how many seconds go by. The sorts of things that change are: anything time-base (like attack and release) and anything frequency based (like the cutoff frequency).
Typical samples that are used in Hydrogen are: the sound of a single drum hit, the sound of a single cymbal hit, the sound of a single cowbell hit. Whenever you put a note in the pattern (or play a note using MIDI), Hydrogen will play whatever sound you have loaded. So, to put together a drum kit you need to gather short recordings of the bass drum, each tom, each cymbal, the high hat open, the high hat closed, the snare drum (snare on), the snare drum (snare off), rim shots, etc.
However, there are no rules about what a sample can be. It's not uncommon to use Hydrogen to trigger non-drum sounds like: audio clips of people talking, a clip from a song, sound effects, audio clips from movies, and famous people speaking. Be creative!