novice user sonar proffessional: cannot hear previous tracks when recording new track any

Mixers don't make very good audio interfaces because routing becomes more complicated. With a normal dedicated audio interface you simply use the direct monitoring and  computer/ input blend to hear the input and the computer playback in your monitors or headphones. 
 
With a mixer you need to avoid the playback leaking back into the stereo USB output or you'll get a loop back. 
Most USB mixers have a dedicated channel strip and a AUX send to the headphones or a solo option. Some mixers have direct monitoring toggles. Because you do not specify which mixer you are using I cannot be detailed. But generally the process is to first get the playback in your headphones using either SOLO or the Aux sends. Some mixers give you monitoring selection.  Your answer is in the manual for the mixer. We can only guess or read the manual for you,, your choice.