Apply the System39.ini volume valancer levels to audio output, so the
Volume controller dialog actually adjusts per-channel volumes. PCM scales
its chunk volume by the channel mapped via the .wai file, and BGM scales
its music volume by valancer channel 0.
Fixes#62.
Used in Ore no Shita de Agake.
Since the musbgm_* interface does not allow multiple channels, this
implementation uses musbgm_* for channel 0 and muspcm_* for channel 1.
kichikuou