There are dozen of on-line Sanskrit-English Dictionary on the internet
(http://www.alkhemy.com/sanskrit/dict/). They are Romanzized Sanskrit with
International Phonetic notation. Basically they're based on the Sanskrit-English
Dictionary edited by University of New Delhi. I'm in Hindu tradition (SeBaba),
whenever the priest read Sutra or recite mantra, they use Sanskrit. They alway
refer to that (or similar) Dictionary, so I think the pronounciation are offical.
As a matter of fact, Sanskrit was abolished as offical language just a couple of
hundred years ago because the confusion of the meaning (one word has 2 to 6
different meanings is quite common) but still is used in religion now.
We've to thinak the British who Romanized the Sanskrit text, because English has
26 alphabets, basically no sound it can not produce. Chinese (Mandarin) has only 5
vowels that is why whatever mantra quoted in Chinese text, about 20% sound
(eg. "r","h") are lost (Sanskrit has 7 vowels). So alway stick to the Romanized
Sanskrit with International Phonetic notation, they're standard, no confusion.
Search in India's Yahoo, you will find dozen of free Sanskrit introduction
lessons. Sanskrit alphabets like English alphabets in a way that it starts with a
few vowels then generate another that why if you mis-pronounce 1 alphabet you will
collide with other. So you have to pronunce ALL 26 alphatbet right first.
Here is one EXAMPLE of The Great Compassion Mantra in Romanized Sanskrit, every
single word can be found in the Sanskrit-English dictionary with proper phonetic
notation (if u trust U. of New Delhi):
http://www.geocities.com/mahayanazengong/compassion.html
The sound track is recorded by the Dharma Realm Buddhist Association (DRBA) which
is founded by The Venerable Master Hsuan Hua in San Fransico, USA (Thousnad
Buddhas City). Because in Sinoyana (Chinese) tradition, the original Sanskrit text
are coded in Chinese text then read in Mandarin. If 20% of sound were already lost
in Tang Dynasty when the text was 1st transcribed, what do you think we use today
Mandarin to read 1,400 yr text. But this is the way Sinoyana still do today while
offical Sanskrit dictionary is just in front of our nose (thank to the internet).
My final suggestion, alway check the (on line) Sanskrit dictionary, if you can't
find it, probably it is an transcription eror because the monks didn't know
Sanskrit very well. With no printing technology, very little books around, our
ancestor didn't 've much reosurce as we have today. Remind that Buddhism might not
be practised in India, but is still India's national treasure. You will surprise
how many Buddhism scholars are still around in India to help you on the internet
today. Because one Sanskrit has 6 meanings is common (eg. arahant), that why there
are so many versions of The Great Compassion Mantra.