AudioMixer output volume

1. In editor

Call this method by reflection:

 // UnityEditor.Audio.AudioMixerController
 internal sealed class AudioMixerController : AudioMixer
 {
     public int GetGroupVUInfo(GUID group, bool fader, float[] vuLevel, float[] vuPeak);
 }

vuLevel and vuPeak are 9-element arrays which get these values - one element per channel

2. At runtime by OnAudioFilterRead

Works only on the level of AudioListener (all sounds) or AudioSource (single sound), no support for mixers or groups

This is better method than 3., because it's called directly from Audio thread and gives you data at appropriate time

 public class AudioFilterMeasureVolume : MonoBehaviour
 {
     private void OnAudioFilterRead(float[] data, int channels)
     {
         // Process data
     }
 }

Process the data like this:

https://answers.unity.com/questions/165729/editing-height-relative-to-audio-levels.html

3. At runtime by AudioListener

Works only for master output data, cannot get data for individual mixers or groups

Call AudioListener.GetOutputData and process the data like this: https://answers.unity.com/questions/165729/editing-height-relative-to-audio-levels.html

4. At runtime by Native Audio Plugin SDK

It's possible to write native audio plugin, which will extract this information: https://docs.unity3d.com/Manual/AudioMixerNativeAudioPlugin.html