How to calculate Mean Opinion Score (MOS) from objective data for
voice networks? MOS is purely subjective based on the user feedback,
but there are some vendors that are able to calculate it from objetive
data such as delay, jitter, jitter standard deviation, and packet loss
of a Voice over IP (VoIP) network. I appreciate if somebody can give
any pointers in finding this information.

There are various objective measures that seek to correlate well with the MOS.

One that I rely on is PESQ:
http://www.pesq.org/

It is based on many of the properties that you mentioned (jitter,
delay, noise, etc)  Specifically, I rely on Agilent's VQT technology:
http://we.home.agilent.com/USeng/nav/-536885778.536882651/pd.html

Be wary though about objective measures.  Most of the time, quality
does not equal intelligibility.  The latter is something that is
harder to quantify, but is quite valuable.