My first question is to anyone who has ever been to a live concert: have you ever heard an artist complain about monitors? In particular singers? This is due to the fact that it is near impossible to sing or play many instruments without being able to hear yourself back. The way a singer determines their pitch and performance is based off of how they react to what they hear back! There is a reason artists walk off the stage over a bad monitor mix. If you cannot hear yourself, the way Beyonce sounds in the leaked video, is how anyone will sound - including your favorite singer - in the same situation! 

While that points to the fact that she may have had a bad day, how do we know she can really sing? It is very simple, the second YouTube clip - the one the audience actually heard - is her actually singing. How do I know? I have recorded many performances like this in my years as a Producer/Engineer. Don't see them on my credits? Labels don't exactly like you bragging about recording "Lip Synching Tracks" for their artist! It is now policy for many artists, to prevent errors if they are going to be performing in front of a national audience. This practice takes out "Insurance" that the performance will be flawless. We have all seen the controversy that ensues, when someone sings poorly on live television. By having the band play to a backing track and playing the vocal from the backing track source, you are ensured to have a great performance, unless the tech puts on the wrong backing track and the band plays the other song. The vocal that plays from the backing track (at least the ones I have recorded) are a punched-in performance that we do not Autotune (some engineers may apply a light tuning but we know we cannot put on an audible amount of it), though we do punch in the performance to sound as "natural" as possible, while still being polished. What the crowd heard is Beyonce singing, there may be some punching and editing done but we are not hearing Autotune flying everywhere, it is what she sounds like, and what everyone who is performing to back tracks sounds like. they are singing to a backing track in fear of a PR nightmare. Looks like they got that anyway!

Can I prove with 100% certainty that Beyonce can sing? Nope! Not after Milli Vanilli. Though, this leaked audio does not prove she cannot sing either, it just shows how terible a talented artist sounds when they cannot hear themselves.