I have verified the GSX folder and all sounds are present but not played.
Since boarding sounds are selected randomly, it's possible you don't have ALL of them (because you haven't downloaded the vehicles installer or latest full installer), so the session has selected a sound that is not there.
Check you have 52 .WAV files in the fsdreamteam\couatl\GSX\sounds folder, with 9 for boarding and 14 for deboarding. If you have less, you haven't updated correctly so either either download one of the vehicle updates, or the full GSX installer to fix this.
If you have all sounds, and you don't hear any deboarding sound, be sure you are inside the cockpit, since they are not heard while outside.