UsbPlayerTest.quieter_10_Test Forth Case

src/main/java/device/usbPlayer/UsbPlayer.java

@@ -136,11 +136,13 @@ public class UsbPlayer implements Device {
     }
 
     public void quieter_10() {
+        if(this.soundWas == -1) {
             Lautstaerke -= 10;
             if (Lautstaerke < 0) {
                 Lautstaerke = 0;
             }
         }
+    }
 
     @Override
     public int getVolume() {

src/test/java/device/usbPlayer/UsbPlayerTest.java

@@ -240,10 +240,17 @@ class UsbPlayerTest {
         UsbPlayer up3 = new UsbPlayer(USBDrive);
         up3.quieter_10();
 
+        //forth case
+        UsbPlayer up4 = new UsbPlayer(USBDrive);
+        up4.setLautstaerke(11);
+        up4.mute();
+        up4.quieter_10();
+
         return Stream.of(
                 Arguments.of("Volume was 10, should be 0 ", up1, 0),
                 Arguments.of("Volume was 50, should be 20 ", up2, 20),
-                Arguments.of("Volume was 0, should stay 0 ", up3, 0)
+                Arguments.of("Volume was 0, should stay 0 ", up3, 0),
+                Arguments.of("Volume is muted,no change can be made ", up4, 0)
         );
     }