You've heard of looping right? Well, this is the multitrack version of it. You just keep layering it up until the song is done. This is how all music was made in the past, the technique becoming popular during the first presidential term of Les Paul and his vice president Mary Ford.
Remixing is hell.
...
And, of course, that's a joke and not a very good one.
Let's make this a lesson in debugging / problem solving...
(And stop me if I failed to understand your situation.)
Your record the first track, drums, let's say. Now you record a bass track. If you solo the bass track, you hear drums, too, when you thought you'd only hear bass.
If you then arm a third track and record, say, piano, when you solo the piano track you still hear the drums and bass -- they're comingled on the track that should have only the piano.
Am I right so far?
If so... what can we deduce (or infer or what-the-heck-ever)? At the most basic, we know that each new track includes the previous -- which is undesired behavior.
Ergo (which is latin for
so), it appears that we are somehow routing the previously recorded tracks into the new track.
So, er... ergo, we trace backwards.Look at the Sonar channel input. We think it should say something like LINE IN or AUDIO IN. If it does -- especially if your card is a SoundBlaster or compatible -- take a look at the Windows audio mixer (click on the little speaker int he system tray.) Click on Options / Properties.
On the Properties dialog click the Recording radio button. In the window below you'll see a list of possible inputs. The checkboxes next to them select them for the RECORDING mixer (different than the PLAYBACK mixer). Select the ones that you think you
might be using to record -- you can narrow your selection on the mixer.
Now click OK. You should see the RECORDING Control mixer. In more or less the same spot on this mixer that the mute checkboxes are on the PLAYBACK mixer you'll find checkboxes to SELECT the input you'll be using for the current session. Make sure WHAT U HEAR is NOT among the selected for our purposes here. If it was that explains your problem right there.
If so, you'd picked a setting for your soundcard's analog built-in mixer [a standard feature of SoundBlaster compatable cards] that essentially routes the output of the monitor section of the soundcard into the input of the audio recorder section so that it records "WHAT YOU HEAR" (hopefully without ear-splitting feedback.)
If that wasn't the case, the trouble is most likely arising in your signal routing in the real world -- most likely in your mixer.
You need to keep tracing the signal backwards from the LINE IN of the computer sound interface. In most cases, if your recording a single source, say a mono or stereo keyboard or drum machine -- you should take your recording signal from the DIRECT OUTs on the mixer channels that the keyboard is plugged into.
If that's not the case -- or you're mixing together multiple keyboards, drum modules, etc, you're undoubtedly taking your signal from either a submix or the main mix outs on your mixer. You need to make sure, if that's the case, that you're not inadvertantly folding your earlier tracks into this mix.
Obviously (at least I hope it is, by now), if you were, this accounts for their presence in each new track you record. You'll need to find a way to monitor your mix while keeping those old tracks out of the signal/mix you're sending to the sound card LINE IN. (See the bit about DIRECT OUTs above)
You should have discovered the source of your problem by now -- but if not I suspect you see the method by which we backtrack and trace the problem. When in doubt follow the money... (oops, that's politics)... er, signal. Same diff, anyhow.
I hope this helps... here's some supplemental reading:
Recording Resources 
