How do you plug in the headset? When I use the headphones, plugged into the "Headphone" port, the internal speakers are disabled automatically. I wonder, why you or your wife are stll hearing the speakers. Does the port not work properly? Have you tried to move the plug for a better contact?
Or do you set the volume so high, that your wife still can hear the sound just from the headphones?
I do not have an USB headphone to check this, but with my USB microphone I select the input source in the System Preferences "Sound" panel. I expect the same should work for the output channel.
What do you see, when you open System Preferences and select the "Sound" (or "Audio") panel and click the Output tab. You should be able to select a device for system output.
For example, on my system I see only the internal speakers, but your headset should also appear, unless your device needs a special driver.
Hello again LeonieDF,
I tried that and it worked fine in iTunes but did nothing for GarageBand, GarageBand still played through the internal speakers. However, I opened preferences in GarageBand and was able to make a selection there and its playing through the headphones and not the internal sperkers.
I appreciate your help,