Overall volume mixdown too quiet

There's some useful ways to self-master a demo, and I'll give you a few minor hints here:

1) I don't use Sonar for Mastering (I prefer Wavelab), but we can do a few things that will help. 
2) Nothing can totally replace hardware, but we can get some good results if we try hard. Get some "good" plug-ins. Waves Tape, L316, and SSL Buss compressor comes to mind, and a great Linear-Phase EQ (I wish Algorithmix would hurry up with 64-Bit, but DMG Audio EQuality is good).
3) If you don't need EQ at all, great! Otherwise, minimal EQ only, please (wide Q).
4) Use a compressor with a very controllable attack/release. I personally don't care for automatic attack/gain controls. This can be first or after the EQ (if you need EQ) try both sequences and see. In compression, go for an attack of about 15-30ms or so, and a Gain of a few dB. Release can be pretty fast, but not too fast! We need a slight pump action going, but nearly imperceptible to the human ear. 
TIP: Careful use of the Waves Tape plugin (without the Monitor control) is worth a look. It's pretty great for a few more dB.
5) Place the Waves L316 limiter last. Try setting the Release character to Warm Analog. You'll get a lot of volume out of that, and ironically, more punch and power than the Punchier L3-16 release settings. Set the release slider to about 30 ms (approx.), then pull down the double-arrow in the center of the two main controls until you start to hear some 'slight' destruction. Then, bring up the Output level to -0.1. Next, enable the lowest band in the L3-16 and grab the priority level for deep bass down a few dBs. This will keep the kick and bass in check. 
6) Tweak and play. 

Lack of good metering in Sonar makes it a really bad choice for Mastering, but for your purposes, it can be OK. You can check your levels in the CakeWalk Limiter plug-in (I forget the name), since it has an RMS meter. 
Also, there's a few "ok" plug-ins Cakewalk offers, but I find the Waves plugs far superior. If you don't have the Waves plugs, or something suitable, like Ozone5, check them out. They're not "bad".

Hope this helps!!

Welcome to the club. There is no short answer. Slamming your mix with a brickwall limiter will (short term) "make it seem" louder (and it will indeed be a bit louder), but there is SO much more to it. Subtractive EQ is your friend. Find out which frequencies (on which tracks or busses) are "pushing" the mud factor. (Hint: it often isn't the ones that "seem apparently" the loudest.) Do lots of cutting in the 350 htz range. Get your mix to be in the ~ -15 RMS range WITHOUT any master buss limiting first. THEN (and only then), try something like Ozone 5. And be gentle. What seems like "ear candy" at first listen will become headache-inducing at repeated listenings. Get the mix to be UNBELIEVABLY good first. Only then play with limiters (gently). It will take time, but you will get there. Have fun! : )