new 2nd 2-stems pretrained model (speech-voice|music) . [Feature]

## Description

i'd like to ask about a new 2nd 2-stems pretrained model , - spliting for speech-voice and music! It's highly wanted for audiobooks, - very often needed to split audiobook's stream! As for personal use for example only for for keeping speech-voice of an audiobook , and also for sites to let user on/off speech-voice/music separately!

The issue requests a new pretrained two-stem speech-voice/music model but names no files, tests, or entry points. Start by locating the existing pretrained-model and audio-separation workflow in the repository. Done would require a trained and integrated model that separates speech-voice from music, with validation showing the requested behavior.

Written by the indexing model from the issue text.