Depending on what your system is doing, it give priority to different programs. For instance, when I pull up Mumble, my Firefox ends up muted (so no YouTube) until I fix it. This might not be your problem but it's worth checking. If you have windows, you'll want to click on the little speaker icon in your task tray. In the window that pops up, look at the bottom and there should be a link that says "Mixer". Click on that and check your system volume levels for internal speakers, external speakers, etc. Also make sure you open up your music player and check that the volume on that is not muted.