"Paul Ryan was correct when he did the tax code. What’s the first thing he decided we had to go after?" Biden said, with a slight smirk. Biden then leaned into the microphone and said in a deep menacing voice: "Social Security and Medicare." 

heres the thing though biden does that whispering thing a lot. He usually does it when he’s “speaking forbidden, politically incorrect truths to the Left,” not when he’s being sarcastic. Furthermore, Grim observed that later in the same speech Biden says that Medicare and Social Security “can stay, it still needs adjustments, but can stay” — adjustments being DC-speak for cuts.

but you should watch the entire 4 min clip. In its context, it's clear Biden is criticizing Ryan for cutting taxes for the wealthy.

it's not clear what hes doing there and in the end of video he says it again Medicare and Social Security “can stay, it still needs adjustments(cuts)

The politifact article, however, doesn't say anything about a video being doctored.

the biden campaign lied about the video being 'doctored'. thats crazy loaded word to use in the era of disinformation. politico and the NY times confirmed that the Biden campaign lied about the video being 'doctored') the biden campaign needs to apologize for tossing around fake accusations.