Make GlobalVolume change running audio

[janhohenheim]

What problem does this solve or what need does it fill?

In a settings menu, users want to change the volume of music that is currently playing.

What solution would you like?

GlobalVolume already exists, let's make also affect currently playing volume :)

What alternative(s) have you considered?

Make the user write this boilerplate or move it into a third party plugin.

Note to any interested readers: this link shows you an example of how to implement this feature :)

Additional context

This issue came up while implementing a settings menu for bevy_new_2d