Sound through wall / Audio zones

This is a theoretical solution. I haven't tested it yet and you have to play around with the distance values.

 // C#
 public class OccludableAudio : MonoBehaviour
 {
     private Transform m_MyTrans;
     private AudioSource m_Source;
     
     private float m_MaxDistance;
     
     public Transform Listener;
     public float OccludedDistance = 5.0f;
     public float FadeSpeed = 10.0f;
     public LayerMask Mask;
     
     void Start()
     {
         m_MyTrans = transform;
         m_Source = audio;
         m_MaxDistance = m_Source.maxDistance;
     }
     void Update()
     {
         float target;
         if (Physics.Linecast(Listener.position, m_MyTrans.position, Mask.value))
             target = OccludedDistance;
         else
             target = m_MaxDistance;
         m_Source.maxDistance = Mathf.MoveTowards(m_Source.maxDistance, m_Target, Time.deltaTime * FadeSpeed);
     }
 }

You have to attach this script to every audio source that should be able to be occluded. You have to assign your player to the Listener transform variable. Another way would be to make the Listener variable static and assign it from the player's script.

 public static Transform Listener;

 // In your players script:
 void Start()
 {
     //[...]
     OccludableAudio.Listener = transform;
 }

This way you only have to assign it once.

Don't forget to setup the layermask to specify what colliders should occlude the audio.

You could set up some, say, Box colliders and use them as proximity detectors to tell the audio to attenuate accordingly. You'd want to change the volume the player hears rather than the source if it's multi-player (or at least, not propagate the volume to other players).