No music on speakers while recording

I work with focusrite Scarlett.
I’m trying to record my voice to a karaokesong, which i imported to Audacity. The moment i click on the recordbutton, ik cannot here any music, either from speakers nore headphones. It is possible to record my voice, no problem there, but it is not working to sing without hearing music.

I hope someone can help me.

[u]Here is a tutorial[/u].

Note - You’ll need to use headphones so the backing track doesn’t feed-back into the microphone and create an echo.

In other words, turn on Overdub.

Here is a link to the Tutorial: