Make GlobalVolume change running audio

## 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](https://github.com/TheBevyFlock/bevy_new_2d/blob/3b236485b5459eb4032a151dc3fae49be7b306ec/src/audio.rs) 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](https://github.com/TheBevyFlock/bevy_new_2d/pull/374)

Start by reading the GlobalVolume implementation in Bevy and compare it with the linked src/audio.rs example from bevy_new_2d. Trace how currently playing audio is managed, then verify that changing GlobalVolume updates active music as well as future playback.

Written by the indexing model from the issue text.