You do have a few options, but they do depend on how the thing was mixed and what the mix contains. If the voice is a narrow range, frequency wise, with few instruments in the ‘hole’ it has in the mix, then you could use selective eq to give to a bit more prominence – but if there is another sound source in that range – perhaps 300-2.5KHz or so, that comes up too. The other possibility is looking at the stereo field. Adding left and right together with one inverted will if the rest of the insurgents are spread left and right, partially or maybe greatly remove the centre vocal. If you then create a mono version summed and then add these, with one inverted you might expose it a little more. This isn’t something you can do within a typical video editor, and success depends very much on how it was created and what it contains. It can also produce unwanted artefacts from the matrixing process – so sometimes the poor mix does at least sound good!
