Acrobat has one ( or more) Achilles heel(s) when it comes to digital signatures! It is the inability of an acrobat document to accommodate valid digital signatures from two or more people on a single page ( think of a legal document where three parties are signing. In medical practice, this is a big headache as more than one person signing off is common( example a doctor and his assistant) . I was told it is possible to do this with third party tools but it is very cumbersome.
Here is a novel idea. Why not allow digital voice signature right into the acrobat signature module? For example When I dictate a document using ViaVoice pro I can create a digital voice signature which shows who signed the document, where and when(not yet possible with Dragon!). While the metadata for the signature can be embedded in the signature to track the creator and time, the validity of the signature can be further enhanced by embedding the unique voice print of the user right into the signature on the acrobat document.
This will provide one other major benefit. Documents with digital signature can now be secured and mailed. In the versions and iterations of Acrobat now marketed, a document containing a digital signature cannot be encrypted! This is a major pain in my practice which runs on Acrobat. I have to strip off any digital signatures and then save the copy to give my patients or referring docs. .
Second, your suggestion about voice signatures sounds like a great idea. There are third parties today that support biometric signatures by embedding a lot of information about a captured wirtten signature (from a signature pad) into a signature. But I'm not aware of any that do it with voice as yet.