Hi Jack! Since the newer versions of Android rolled out, volume throttling via software became a thing. There is a possibility that this is what's happening with your HTC Desire 601. To get around this, just press the volume up button repeatedly until you see a pop-up dialog on the screen warning you about the dangers of raising the volume up too high. Just click OK through to confirm this and you can make the phone sound louder than it does with the volume limit on. Try it and see if it works!