Transaction

Unless your uncle doesn't have any other proof of his money not having been paid back, or unless the person to whom the money was initally transferred doesn't himself admit to defaulting, there isn't much that can be done. This isn't the only instance where an innocent person is caught in messy proceedings because he/she left their genuine signatures lying around which got misused by someone else.

voice recording may or  may not accept as legal proof in the court of law. still you may approach court for the same

You need to prove that the person borrowed from you and did not pay.

Since the person has declined to pay it shall be appropriate to get proper legal opinion after showing all related docs/communications from a very able local senior counsel of unshakable repute and integrity specializing in such/civil matters.

Why should anyone sign on blank paper?

If amicable settlement is possible try by involving elders of the family.

Or approach a very able counsel and let court decide on merits.