start at 50% volume for all channels to allow for more user control

src/main.cpp

@@ -64,9 +64,9 @@ static Sint32 window_height;
 
 static Mix_Music *music_setting_off_piano;
 
-static float volume_master = 100.0f;
-static float volume_music  = 100.0f;
-static float volume_sfx    = 100.0f;
+static float volume_master = 50.0f;
+static float volume_music  = 50.0f;
+static float volume_sfx    = 50.0f;
 
 static bool Running = true;