It is quite unlikely that Google does maintain any dictionary of its own. They are in the business of online advertising and not English language. (Though, of course, the language itself would hold immense value for their principle offering.)
However, meaning and definitions on Google can be obtained by searching
define: <word or phrase> command in their principle search engine. The results returned are bot (automatic) based aggregations from various well known dictionary websites and/or products and not essentially populated by Google.
You will need to refer to the practices adopted by the original owner of the dictionary content. This wiki entry notes:
- Collins COBUILD
- Oxford American College Dictionary
- Oxford Pocket Dictionary
as some of the sources used by Google for its dictionary.
The phonetics notation used will, therefore, depend on the word and from which provider it is sourced, Sadly, there is no way, as of now, to know that.
While Oxford is known to be IPA conformant, This wiki entry notes that COBUILD does deviate from it. You will need a little bit hard work and hard guessing, I guess, but you should get hold of it sooner than later.