Apparently what's happening is this: The USB headset has a built-in sound card while your speakers are connected to the motherboard's sound circuit. When plugging the USB set in, it automatically takes the role of a primary sound system, thus muting your speakers.
Happy news! You can select your devices in Aces High: Open your Clipboard, choose Options-> Preferences-> Game Sounds. Most probably all of the four slots will say something like "Windows default". Now choose the USB device (maybe the brand of your headset) in the Headset drop-down menu and the motherboard sound circuit (often Realtek) in the Speakers menu. Click OK and that should do it.