cегодня is derived, like many other Russian adverbs, from a noun phrase,
cего дня ("[on] this day"). Both words in this noun phrase are in the Genetive case, the first one having a typical pronominal declension endinhg
-его, in which
г is written, but
в is pronounced:
сего - pronounced as
его - pronounced as
синего - pronounced as
(the same rule applies to the variant of this ending,
большого - pronounced as
The pronunciation of the letter
[и] in these words is due to the rule of the reduction of unstressed vowels. According to this rule, the letter
e in the syllable that precedes the stressed syllable is pronounced as
So, the IPA for
cегодня is [sʲɪˈvodʲnʲə], practical Russian transcription is [сиˈводьня].
Also, you can listen to the recording of this word pronounced by a native speaker of Russian.
Pronunciation with [г]/[g] is incorrect.