Fix #7189: Fluidsynth volume gain too high

src/music/fluidsynth.cpp

@@ -149,7 +149,11 @@ bool MusicDriver_FluidSynth::IsSongPlaying()
 void MusicDriver_FluidSynth::SetVolume(byte vol)
 {
 	/* Allowed range of synth.gain is 0.0 to 10.0 */
-	if (fluid_settings_setnum(_midi.settings, "synth.gain", 1.0 * vol / 128.0) != 1) {
+	/* fluidsynth's default gain is 0.2, so use this as "full
+	   volume". Set gain using OpenTTD's volume, as a number between 0
+	   and 0.2. */
+	double gain = (1.0 * vol) / (128.0 * 5.0);
+	if (fluid_settings_setnum(_midi.settings, "synth.gain", gain) != 1) {
 		DEBUG(driver, 0, "Could not set volume");
 	}
 }