If I remember correctly, the Parallax PIR's data pin goes high when the detector is triggered. According to the playground entry they do go low from time to time, randomly, but that shouldn't matter. Good MIDI practice is to send a "noteOn" event with detection, add a short delay (delay(10) or delay(20)) to prevent multiple triggering, then as soon as the pin returns a LOW again send the corresponding MIDI noteOff event.
A way I did this on my own little RF-to-MIDI project was to set an internal flag, "sounding," whenever I sent a noteOn event; this was a reminder to the circuit to send a corresponding noteOff the moment the trigger was released.
On the sampler side, set yourself up in one-shot sample, drum mode (whatever your particular hardware/software calls it) so the entire sound clip will play regardless of the length of the triggering note event.
Oh, let me pseudo-code this;
if PIR = "HIGH" then
noteOn;
sounding = "HIGH";
delay(20);
if PIR = "LOW" and sounding = "HIGH" then
noteOff;
sounding = "LOW";