Aha you have stumbled across the Achilles heel of the Esi range.
Within the innards of this Sampler is a little connector and a lead.
One end of which terminates at the headphone board and the other can be plugged into one of two places.
The first is a duplicate of the main outputs situated on the main board.
The second is a duplicate of the Effected outputssituated naturally enough on the effects board.
I would hazzard a guess that it is currently in the Effected position.
Which does not allow you to hear during sampling.
If you change the position to the main board you should be able to here while sampling.
But you will not be able to hear via the headphone socket the effected output.
I think this information is correct as i remember fiddling with mine a few months ago to find out the problem.
Possibly your best bet for headphone monitoring is to aquire a patch bay and a Headphone amp into which all the outputs are fed.
But i surpose one could even hardwire a switch onto the front panel to select from which source you will be monitoring.
Sadly there is no software function fo this.
I asked Emu and found that this is indeed normal.