You can do it using the “Overdub” feature.
I presume that you are using Audacity 1.3.12, but if not, upgrade to Audacity 1.3.12 first.
Enable “Overdub” in the “Transport” menu. (Software Playthrough must be OFF)
Import or record your music track.
Use the Volume Slider in the Track Control Panel on the left side of the track to lower the playback volume of the track.
Record your vocal track.
The level of the music track may also be adjusted using the track volume slider after you have recorded the vocal track.