Really, the only app that comes anywhere close [if the phase flip trick doesn't work, which it usually doesn't] is Izotope's Music Rebalance
It is only available as part of a larger package & starts at $399
Even at that price it isn't perfect. It works on some sources better than others. A demo of the full RX7 suite is available, so you can at least ...
Most media players can run multiple instances with independent controls. For VLC, for example, just remove the checkmark from Allow only one instance.
That said, you might need to "raise" the correct instance (bring focus to that instance) for keyboard shortcuts to take effect. Use AltTab to select an instance, then apply the keyboard shortcut.
VLC also ...
You have two options, both bindings against software created by the excellent LAME Project.
The lame npm package is a full-featured binding against the libmp3lame C library. I would recommend this unequivocally, except that it hasn't been updated in four years.
You could also use the node-lame npm package. It is a wrapper around the lame CLI tool, ...
I have developed an app that does just what you describe.
Currently it features intervals, chords, scales, and perfect pitch training. There is also a random mode where you can shuffle all the above.
I intend to add more functionality soon.
If you want to give it a try, here it is:
I would suggest Audacity but since you want to work on the files directly I would suggest FFMPEG.
However FFMPEG is complicated and only works on the command line. Here is an example of trimming an audio file:
You can try Audacity. The tool is really powerfull when it comes to audio manipulation, you can work with multiple tracks, export to different audio formats...
The software is german, but as far as I know there are multiple languages available after the installation.
OBS has some push to mute options as do some games, however if your friends are open to discord I would look at setting up a room in a discord server for you as a twitch streamer that you can move your friends in when streaming, them discord has either push to talk or push to mute options, you could configure OBS Pute to Talk and Discord Push to mute to ...