Learn English – Phrase for describing a cable without a connector at the end

Even though a cappella is technically correct, I think the term you're really looking for is vocals.

If you're trying to isolate the vocal track from the instrumental tracks of a song (which I think is what you're doing), the result can be called: an isolated vocal track.

You can use these terms: isolated vocals, isolated vocal tracks, vocal track, vocal only songs, as a search term on most audio/video websites on the web.

"More likely than not" logically means with a probability greater than 50%. A probability of 50% would be "as likely as not".

But the user of the phrase is not making a mathematically precise estimate of probability. They are expressing what they think is likely in an intentionally vague way, and it's misplaced precision to try to assign a number to it.

As an opposite, one could simply say "unlikely" to mean a probability of less than 50%. Or, given a proposition A, one could say "It's more likely than not that A is false."

I guess you could say "less likely than not", but I've never heard that.