Richard van Straten made a very useful manual for YEM version 2.3.2
(manual version 1.2.1) in 62 pages.
This manual is in Dutch.
If you can't find it somewhere you may send me an e-mail
so I send you this manual in appendix (pdf).
Free traduction of the answer to your question,
from the manual from Richard van Straten :
"....we need specific software called "Loop Editor" to let
the sound continue,.....most of the audio-editors (DAW) can
make a loop, but that is not the loop we need because it makes
no samples with a loop.
A "Loop Editor" makes in the sample (WAV) two indexpoints
that can be read by the Tyros/Genos to define the start and end point
to continue the sound as long as the keys are pressed,....."