Did you ever think of a similarity between "woman" and "violão" ? I consider the [w] as in woman [wɪmɨn] or well [wɛl] to be quite similar to the one in violão. Just a random thought.
You could, in order to improve your pronouciation, either talk to a native-speaker, or listen to the pronounciation of these words online, and try to imitate it. Once you get the hang of it, it's pretty easy. Até! :)