When you plug in a USB sound device, it's not the same as a headphone jack. It's like plugging in a whole new sound card.

Somewhere in the operating system you will find a setting to control which output is the "default" audio output for the operating system. It will let you choose which sound card is the default one.

Don't know how that's handled on the mac. In Windows you right-click on the volume icon in the taskbar and select "playback devices", where it gives you a list of all the sound cards installed on the system. From there you select one and set it as default. I'm sure there's something similar in the Mac's control panel, somewhere in the audio devices configuration.