The open-source code for Spleeter, a source-separation algorithm, has been released by the research team at Deezer, a music streaming service — and early experiments with it show it to be pretty effective at (for example) separating vocals from mixed mono or stereo tracks. The code can be run under Linux, Mac OS or Windows, but you have to use command-line instructions to make it work — there are currently various options to demix content into vocal tracks and 'everything else' and also four- and five channel demix options that seek to break tracks down into drum parts, bass, vocals and 'everything else' and drum parts, bass, vocals, piano and 'everything else'.
As with all such tools, the separation is never perfect, and works much better on some tracks than others, but it does merit investigation, and according to Deezer, the models that underpin the process are being continually refined, so Spleeter should improve over time. Since the announcement, more user-friendly web-based front-ends using the code have already appeared at https://melody.ml and https://moises.ai. Check it out!