I can't Rec launching a musical base from audacity

Good morning. I’m new of the Forum and I’m Italian.
I have a big problem with the use of Audacity. A few days ago I bought a Behringer mixer because I would like to Rec using Audacity on my portable.
First of all I connected the mixer to the laptop by usb port.
I loaded the drives for the Audacity to recognize the mixer. All OK!
Then I imported, in Audacity, a karaoke base on which to sing. The microphone is connected to the mixer.
Then I found my first problem. After checking the levels of the base and the microphone I started recording to start the base and
instead of giving me a second track where to see my voce coming from the microphone, this my voice followed the karaoke base in the same track without to listen any sound.
Can you halp me please?
Thanks in advance.

To record to a new track, hold down the “Shift” key while you click the Record button (or use the shortcut “Shift + R”).