You have background sounds, system sounds, and other families of sound components that could be activated from within a Web page. That synchronization can be a function of how fast the audio is displayed. Historically, background sounds are distinct from multimedia presentation audio tracks. Global control of volume is an important special case of this. Whatever is being used to play that would have to control the volume independently.
Some pages use real audio as background sounds.
This may cause my speech synthesizer or UA to crash. Different from loading. Ask MS Windows media people about possibility of independent volume control of audio sources. The consensus is that there are so many ways people need to change colors and fonts, we allow them total control. In IE, they pick up whatever the system offers.