The word Совет seems to always be transliterated in English as Soviet. However, it seems to me that it would be more appropriately transliterated as Sovyet.
I think this is the most logical variant as there are three syllables when Soviet is spoken by a native English speaker, whereas Совет and Sovyet each only have two syllables. This also seems to more accurately reflect the Russian pronunciation of the word (as I understand it).
I'm curious if there's something that I'm misunderstanding, or if this is just a result of some arbitrary difference between transliterations that has been simply been standardized over time, while some other words have not (e.g. tsar vs. czar).
Edit
Ideally, I'd like to find the etymology of the word Soviet that would include references to various historical usages and/or information about alternative transliterations.