1. Have you tried a different USB cable? Faulty USB cables can cause connection/power/audio issues so this is very important. The cable used is a standard USB A to B cable and is used in USB MIDI keyboards, printers, scanners. Most people should have access to one.
2. Is your unit connected using an external USB hub at all? If so please remove this and try connecting your unit directly to a dedicated port on your computer.
3. Have you tried all the USB ports on your computer? This is important as each will perform differently depending on how each port is internally hubbed.
4. Does the green USB light come ON on the front of the Scarlett Solo when it is connected to the computer?
5. What driver is assigned to the device in the Device Manager? To access this, go to Start Menu > Control Panel > System > Device Manager. Please expand 'Sound, video and game controllers' and right click on your device > Properties > Driver Tab > Driver Version. Please check to see whether the assigned "Driver Provider" is Focusrite Audio Engineering. If it isn't then this could well be the problem. Similarly check there are no warning signs next to the device.
6. The easiest way to check whether there is a fault with the preamp is to plug a microphone or a guitar into one of the inputs and either speak into the mic/play the guitar with the gain dial at 12 o'clock. You should see the gain dials glowing to indicate that signal is going through the device. Similarly if you then plug headphones in and switch the Direct Monitor switch to ON you should hear the mic/guitar in the headphones when you play and the Monitor dial is sufficiently turned up.
7. Providing these previous steps have been successful, when you select the Scarlett Solo as the Default Playback device in Control Panel > Sound and then play audio from Youtube/iTunes through the Scarlett Solo with headphones or speakers plugged in, you should be able to hear this audio.
If you were able to achieve or answer yes to all of these questions then the issue that you are having is going to be a software rather than a hardware issue. If you answered no to any of these then it is possible that there is a hardware issue with your Scarlett Solo. For further clarification please contact Technical Support.