The request seems misplaced as far as RoboHelp goes.
"Voiceover" is something that would normally be within the realm of something like Adobe Captivate, or Premiere Pro. Thinking "motion video" here.
You may be seeing requests for satisfying something known as "Section 508". Also known as "ADA Compliance". And RoboHelp does offer features that assist with that process by allowing the author to specify Alternate or Title Text for things such as hyperlinks or images. Beyond that, you are dealing with basic text on a screen.
Generally speaking, this is reserved for folks that are visually (but not audibly) impaired or blind. And in those cases, the individual typically is running some sort of "screen reading" software. Again, generally speaking, the individual tabs to the different elements and the screen reading software picks up on the alternative text and "reads" in a computer generated voice what the title was that was assigned to the object.