Not being able to adjust the volume is a known issue with the Windows volume mixer in the Settings app.
You can access the old volume mixer, which should work correctly, by opening the Windows Start menu, and typing in sndvol
then pressing Enter.
Note that you might need to click and drag the right-hand side of the window to expand it so all applications are shown (there is no scrollbar, even if some are beyond the edge of the window)
If VoiceAttack isn't visible in that mixer (even after expanding the window), have VoiceAttack play a sound and it should appear.