Update

Jump Game/Assets/Scripts/Menu/Menu_Buttons.cs.meta

@@ -4,10 +4,8 @@ MonoImporter:
   externalObjects: {}
   serializedVersion: 2
   defaultReferences:
-  - MainMenuPanel: {fileID: 8730269676052092079, guid: 95c270678e350c64e9b476d7f28a859d,
-      type: 3}
-  - SettingsMenuPanel: {fileID: 7135312091190565530, guid: 09acfb5a6df9ae1458aea9c3923a799b,
-      type: 3}
+  - MainMenuPanel: {instanceID: 0}
+  - SettingsMenuPanel: {instanceID: 0}
   executionOrder: 0
   icon: {instanceID: 0}
   userData: 

Jump Game/Assets/Scripts/Menu/SettingsMenu.cs

@@ -2,16 +2,20 @@
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Audio;
+using UnityEngine.UI;
 
 public class SettingsMenu : MonoBehaviour
 {
     public AudioMixer audioMixer;
-
-    public void setVolume (float volume){
-        audioMixer.SetFloat("volume", volume);
-    }
+    public Slider volumeSlider;
 
     void Start(){
-        audioMixer.SetFloat("volume", -30);
+        volumeSlider.value = PlayerPrefs.GetFloat("volume", -0.30f);
+        audioMixer.SetFloat("volume", PlayerPrefs.GetFloat("volume", -0.30f));
+    }
+    
+    public void setVolume (float volume){
+        PlayerPrefs.SetFloat("volume", volume);
+        audioMixer.SetFloat("volume", volume);
     }
 }

Jump Game/Assets/Scripts/Menu/SettingsMenu.cs.meta

@@ -3,7 +3,9 @@ guid: 0f70ad7df61e46f4193f65c69cff1ce8
 MonoImporter:
   externalObjects: {}
   serializedVersion: 2
-  defaultReferences: []
+  defaultReferences:
+  - audioMixer: {fileID: 24100000, guid: 5a3abbd28329b73439a81ec45b2d1906, type: 2}
+  - volumeSlider: {instanceID: 0}
   executionOrder: 0
   icon: {instanceID: 0}
   userData: