There are 4 things to keep in mind when using voice translations:

1.  Speak clearly, hopefully without background noise
2. Translator recognizes native tongue better and
3. Make sure the right language is picked as the “From” language
4. Translator works best when translating full sentences. It can make errors, especially when the phrase lacks context.

Voice translations only work when you are online and for the following languages: 

https://translator.microsoft.com/help/articles/languages/