Is there a verb for singing nonsense syllables to substitute for unknown lyrics?

You can say fudge the lyrics.

Example of authentic use:

I discovered recently that there's a line that goes "Just your typical prototype." I usually fudged my way through the lyrics with something having to do with keeping kimonos tied, and wondered why the hell her bathrobe had anything to do with it.

Instrumentalists do this too, but then it's called faking it. You have to do that in certain passages of the last movement of Tchaikovsky's Fourth Symphony.

If you honestly mishear the lyrics then, mondegreen could work. You'd have to mistake actual lyrics for la la la or dum dum dum.

Of course its a noun, so I guess it wouldn't work. I don't think there are any single words for what your thinking of though.