What Stable Volume does
Stable Volume is a playback feature that balances the range between the quiet and loud parts of a video, so viewers are not reaching for the volume when a whisper is followed by an explosion or a loud sting. It works on the whole soundtrack rather than singling out speech, and it is applied during playback on the viewer's device, not to your uploaded file.
Per YouTube's own documentation, it is not available on every video.
What Voice Boost does
Voice Boost is aimed specifically at dialogue. It reduces background sounds and highlights vocals, which helps viewers following speech in a noisy environment or over a busy music bed. Again, it is a playback setting the viewer controls, described in YouTube's Voice Boost documentation, and it is not offered on every video.
Can both be active at once?
No. YouTube's current documented behaviour is that Stable Volume is automatically turned off when Voice Boost is switched on. They are alternatives rather than layers, which is part of why the two get muddled together: a viewer enabling one silently loses the other.
Do these replace proper audio mastering?
They do not. Both features are corrections applied at playback, on some videos, for viewers who happen to have them enabled. None of that is under your control, and none of it improves the file you uploaded.
If your source audio is noisy, unevenly levelled or clipped, playback processing has less to work with and can make the problem more obvious rather than less. Voice Boost pushing dialogue forward also pushes forward the hiss sitting behind it. Clean, balanced source audio is what makes either feature sound good, and what makes the video sound right for everyone else too.
What should creators do before uploading?
- Deal with noise and room sound before anything else.
- Level dialogue against music and effects so speech is always clearly on top.
- Master the whole programme to one consistent loudness with peak headroom left.
- Upload the highest quality audio you have and let YouTube do the encoding.
The practical numbers are in the best audio settings for YouTube. If you would rather not do that pass by hand, Optivox runs it automatically on upload; the podcast mastering page covers the same workflow for talk-heavy video.