The volume setting of the background can be modified by this property.
The actual perceived volume may depend on other factors. If the sound has been digitized at an unusually low volume, you may need to raise the volume setting quite high. This may yield a very noisy sound as you will also be increasing the ambient noise in the sampled sound. Digitizing is a complex activity but you should always strive for the highest possible signal to noise ratio.
Other factors that may affect the apparent volume would be the user preference settings in the computer. There may also be system controls for blending and mixing sound sources and these may be set to unhelpful values.
This property can obviously only control the source volume of the sound generated by the browser.