Better volume scaling in PassThroughUSB example

examples/HardwareTesting/PassThroughUSB/PassThroughUSB.ino

 void loop() {
   // read the PC's volume setting
   float vol = usb1.volume();
+
   // scale to a nice range (not too loud)
   // and adjust the audio shield output volume
-  vol = vol * 0.75;
+  if (vol > 0) {
+    // scale 0 = 1.0 range to:
+    //  0.3 = almost silent
+    //  0.8 = really loud
+    vol = 0.3 + vol * 0.5;
+  }
+
+  // use the scaled volume setting.  Delete this for fixed volume.
   sgtl5000_1.volume(vol);
 
   delay(100);