SCI32: Fix Phant2 movie-vs-game playback volumes

This update should give the game the default mix it receives in
Windows. If necessary, the half-volume audio bug in the DOS
interpreter can be added as an additional hack for this game, since
there are still some sub-par audio mixes that might need additional
correction (like Curtis talking to Blob when taking her out of the
cage in his apartment at the start of the game) which were also
bad in the Windows version of the game.

Fixes Trac#10165.

1 files changed, 11 insertions, 0 deletions

diff --git a/engines/sci/sound/audio32.cpp b/engines/sci/sound/audio32.cpp
index c3f9eb4098..ad9af4ebf6 100644
--- a/engines/sci/sound/audio32.cpp
+++ b/engines/sci/sound/audio32.cpp
@@ -359,6 +359,17 @@ int Audio32::readBuffer(Audio::st_sample_t *const buffer, const int numSamples)
 				} else if (volume > 84 && volume < kMaxVolume) {
 					volume = 63;
 				}
+			} else if (getSciVersion() == SCI_VERSION_3 && volume != kMaxVolume) {
+				// In SCI3, granularity of the non-maximum volumes is 1/32
+				volume &= ~4;
+
+				// NOTE: In the SSCI DOS interpreter, non-maximum volumes are
+				// divided by 8 which puts them in a range of [0, 16). That
+				// reduced volume range gets passed into a volume function which
+				// expects values [0, 32). So, effectively, all non-maximum
+				// volumes are half-volume in DOS in SCI3. In Windows, volumes
+				// [120, 124) are the same as 127 due to a programming bug.
+				// We do not emulate either of these incorrect behaviors.
 			}
 
 			leftVolume = rightVolume = volume * Audio::Mixer::kMaxChannelVolume / kMaxVolume;