Setting the volume on an `AudioInstance` instead of `PlayAudioCommand` doesn't set it relative to a channel.

[Selene-Amanita]

Follow up of #103 / #106

If you have an AudioChannel called my_audio_channel with volume 0.5 let's say.

The fix works if you do my_audio_channel.play(my_handle_audio_source).with_volume(0.2);, it would set the volume to 0.1, as expected.

But if you do:

    let my_handle_audio_instance = my_audio_channel.play(my_handle_audio_source).looped().paused().handle()
    ///...

/// Then in another system
fn other_system (
    mut audio_instances: ResMut<Assets<AudioInstance>>,
    //...
) {
    //... get my_handle_audio_instance somehow
    let sound = audio_instances.get_mut(my_handle_audio_instance).unwrap();
    sound.resume(AudioTween::default());
    sound.set_volume(0.2, AudioTween::default());
}

then the volume will be set to 0.2 instead, I think, I do basically that and when the volume of the channel is 0. I still hear the sound.

[NiklasEi]

Yes you are right, this was an oversight on my end in #106. Thanks for the issue, I will adapt the behavior for setting the volume on an instance.

[PPakalns]

Have the same problem. In my case I have implemented spatial audio manually and need to manually update volume and panning.

It is confusing that there are API interfaces where you need and don't need to manually take into account channel volume.

UPDATE: This requires me to write two different paths for setting sound level upon creation and upon update.

[nixon-voxell]

May I ask if this issue is resolved and how should it be handled now? (I'm using spatial audio as well)