Recording tracks from music on my computer?

File > Import > Audio

...sorry, I mean select Recording devices, right-click and enable "Show Disabled Devices".
Then, instead of just Microphone, you can enable (again, depending on the hardware and drivers that came with your PC), something like Stereo Mix - but that can be different or, as I say, not exist at all.
 
Also, it's a matter of luck how good those drivers are. They have level controls of their own and they must be set so you get good levels to record with, but without actually distorting. One of my PCs the level is never loud enough and on another the level controls have no effect at all. My internet PC works great, though.

Nearly all PCs have some sort of Dolby that messes with the sound, so to turn that off, right-click the speaker symbol on your Windows bar, select Recording Devices, click on Stereo Mix (Realtek High Definition Audio Device), click on the Properties button, go to the Enhancements tab and select "Disable all sound effects".
 
I also found it better when, in Advanced, I disabled "Allow applications to take exclusive control of this device".
 
In the Levels tab there's a second level control. This one gives me different results in every PC. Experiment.
 
And finally, in the "Listen" tab, turn off "Listen to this device". That is to stop it from feeding back on itself when you record.
 
That's about it. Those are my settings. If you search for this on the internet, you will find contradictory advice. As I said, that's because every PC is a bit different.
 
You then open Sonar and go to
Edit/Preferences/Audio/Playback and Recording.
In Driver Mode, select MME (32).
Wait for Sonar to re-scan the available sample rates,
then create a new project:
> File/New/--Blank Project--
In the left-hand empty block, insert a new track
> Press your [Insert] key/Audio track/OK

Drag the bottom line of the new track down to make it bigger. You should see a dropdown somewhere with a -None- in it and a stylized, reverse gray "I" to it's left. Click on that and select "MME Devices", "Stereo Device" (On another PC I tried it said "RealTek devices", so, different all the time).

You are now ready to go.

To get a decent level, you may have to push the PC volume quite high (that's the little loudspeaker symbol on the right hand corner of the Windows bar - mine's on max). To prevent deafening yourself, plug in earphones (just to disable the built-in loudspeakers - don't put them in your ears!)

I have external speakers with volume controls so I have my PC volume on full and can set the volume on the speakers.


HTH.