The premise is a bit strange since the 7k is only a stereo audio device, as opposed to a multitimbral midi device, and Cakewalk a multitrack audio device it should be obvious that you record the backing into two audio tracks in Cakewalk and then overdub your vocals into another track for later mixing.

Naturally with a good quality full duplex soundcard it would be more usual to overdub via the pc. However if you decide the 7k mic input and dsp is better than your pc soundcard mic input you could use the monitor setting in Sound Menu to switch off the speakers/headphones. Now you can feed the backing audio from Cakewalk into the aux input and listen via the 7k speakers/headphones, and record your vocals through the 7k mic input and the usb connection to the relevant track in Cakewalk with no backing on the vocal track.