Since I'm not a dev I'm not sure. But I did see a comment from the rouge firmware dev that it is possibly a hardware issue.
In the Sound Mixer application "AlsaMixer v1.0.26", I can see that the Headphone volume can be controlled independently of the system's master volume. It can be gained from +6.00 to -25.00.
But the speaker volume can't be controlled independent of the master volume, the PCM.
The PCM ranges from 100% (dB gain 0.00) to 32% (dB gain -30.00), but can't go any lower than that, it simply mutes if you go any lower.
This is all that I can see anyway, I'm not all that knowledgeable about sound hardware, so no clue if this is software, or just the software working with the limits of the hardware.