There are a couple of different ways to achieve a fade out. The easiest by far is to use midi controller #7 (volume) -if- your sequencer has the ability to mix down (i.e. record this gradual program change). If your sequencer does not have mix down ability, you can program the patch(s) to use an alternate controller (such as the mod wheel) for midi volume control and record this controller into the sequence before the duration expires.

Another way requires the patch to have release (midi#72) programmed into them and you send this midi control a value of 127 before the duration to the note(s) expire.

A third way also requires programming the sound and simply involves playing around with the envelopes to achieve a long and gradual decay. -I hope this helps.