Does this occur all the time or only when certain programs like skype or your game are running? Sometimes, these programs can utilize ducking in the system sounds, basically turning down all other audio sources to make that program's sound clearer. Helpful when you need it, annoying when trying to watch youtube videos.
Also, have you gone to your playback devices and ensured your level for the main output was turned up? You can do this by right clicking the speaker icon on your taskbar and selecting "Playback Devices." Right click your default audio device (it should be a separate one for your USB headset) and select properties, then select the levels tab and ensure that is at 100 (or whatever you believe is comfortable).