I'm guessing this has been asked a thousand times but I can't find an answer so I'm going to ask...
 
What causes the difference in recording levels and playback levels on a Sonar track?  For example, I just recorded a sample vocal audio track and watched the level meter as I did so.  It pretty consistently stayed around -6db.  However, on playback, the meter stays at around -24 db.
 
What causes the difference?
 
What I really care about is getting a good, strong signal suitable for mixing without clipping.  If there's a tutorial out there somewhere, I'd appreciate hearing about it.  (I have read and understand (I think) the posts that explain that Sonar doesn't control the strength of the incoming audio signal.)

This monkey only know this fact- 
Your interface controls the level.
Setting Sonars track levels does not have any effect on the record level. 
If you turned any of the track levels down while recording that would explain your issue. Just turn them back up. 

If you start with a clean, new project, and set everything in SONAR to 0 db, and you still see/hear a difference, then check the settings on your Audio interface control panel.
Sometimes inputs and outputs maybe set to different db levels (e.g. +3, - 10, etc)
 
Also, sometimes interfaces that offer real time monitoring can screw up levels in the sense that you hear something other than what SONAR is putting out...

Before you playback, turn off the RECORD button on the track and the playback level will appear normal.  With the RECORD button on, you are still monitoring input.
 
Maybe you already know this, but it got me at first.
 
BTW, I always use Shift + Record to turn the  input echo on and off at the same time as record.  In this context, by "echo" I mean listening to the signal coming in.  You don't want to hear the input signal when playing back but you DO want to hear it when recording.  This sounds like a minor thing, but when you are recording 16-20 background vocals one right after the other, it can get confusing if you aren't extremely methodical.  I am sort of rambling off topic, but this may help someone.