Fix UAESND stream volume handling

Fix UAESND stream volume handling

- Apply writes to the current sample set volume register immediately.
  The stream volume was only loaded when a sample set was (re)loaded,
  so volume changes during playback (e.g. AHI volume fades on a
  looping sound) were silently ignored, even though current-set
  register writes are documented to take effect on the next sample
  period.
- Scale samples by the set volume and board channel volume
  multiplicatively. The previous averaging formula had a 0.5 gain
  floor, so volume 0 still played at half loudness.
- Initialize the board channel volumes for all stream channels, not
  just the first two.