Is the voice of a character wearing a helm recognizable?

This depends. How many times have you met the person? Are they good at throwing their voice off a bit? Lots of factors that go into play here.

When in doubt, assume you do not know the person, and ask for a DM. We will then work with some rolls to decide.

Masking your voice will come under the Bluff skill. If you have a low Bluff skill, you will be very bad at this.