I connect my 2012 MacBook pro to an external Schiit Dac via USB. This has worked just fine for years. Yesterday I suddenly had no sound and the speaker icon on the menu bar was greyed out. If I alt click on the speaker icon it shows the Speaker-Schiit USB Audio device in the device list, so I guess the USB port is still recognising the DAC. Just for completeness, some years ago I broke off a headphone jack in the headphone socket, but that's always been fine with me as I've used the USB to DAC to get sound. But no longer. Any help much appreciated.