Mmm, the balancing of the leads does help quite a bit. Glad to see you makin' progress.
Just for shits and giggles, I'mma going to punch this thing up, and I'm going to go through every step it takes to get there. It's pretty much 100% on a whim, so lucky you! Long ass post for a single production tip ('cause it has pictures), so GET READY TO RUMBLE!!
Alright, so first off here's a picture of your current mix - the waveform in all of it's glory. It's pretty cool, since it doesn't have any clipping, nor does it seem to have any compression or limiting artifacts (more on that in a bit).
Look at all of that sound space, just sitting there unused! When there's silence for a solid majority of the track, it often means you need to work on making the track louder.
With a little amplification, we can get a little bit of sound out of it, but not too much. That's because there are a few tiny peaks that cause clipping if you raise the volume too high. Clipping is almost always a bad thing, so we do want to avoid that as much as we can, but there's still so much space that we want to fill...
Upon closer inspection, one can see that this particular point just touches the top - any louder and it'll cause the clipping. We don't want that!
So, what to do? Why not tell the track that there's a limit to how high the maximum volume is, and force the peaks to hit that ceiling? Yeah, hard limit that sucker, using whatever limiter you have! (Audacity comes with a basic limiter in it's "Effects" drop down - that's all I utilized in this picture)
Sweet, no more peaks! We can now amplify this considerably more without actually causing clipping, thus gaining the space that we've wanted all along.
How does this look up close? Well, it depends on how you decided to limit it. There are generally two ways to limit a track - one can either give it a "soft" ceiling, which means the program does what it can to retain the original shape (which simply "softens" the sound at certain points), or one can give it a "hard" ceiling, which doesn't shape the track at all, instead "slicing" the sound file once it hits the ceiling. Both have their advantages, and both have their particular ways of introducing artifacts when you rely on them too much, but with practice it becomes easier to hear what an acceptable level of limiting is.
This is a nice example of soft limiting - it retains the shape, softens the sound at those points so nothing goes above the ceiling
This is a solid example of hard limiting - the peaks become flat once they hit the ceiling. The music doesn't soften, but the artifacts sound like clipping if they become too prominent.
Once the peaks are out of the way, amplifying the sound to its limits gives you a greater deal of sound, filling the spaces almost entirely.
The moments that peaked once before now don't have nearly the difference between parts as it once did. While this can create a homogeneous sound which lacks some of the clarity of an untouched track, often those few peaks contribute little to the track, and instead make the rest of the track quiet, by comparison. In this case, one can easily hear how much can be gained by utilizing limiting and amplification.
That being said, even with soft limiting it's easy to go too far with it, ruining a once solid track. Take a good, hard look at what happens when you have too much of a good thing.
There are no peaks, and everything ends up sounding like a homogenized mess. While following one's ears is always best, there's a good rule of thumb to follow as far as figuring out when you've gone too far.
"When your waveform looks like a sausage, it's cook'd".
When you limit, don't start a sausage party. Limit responsibly, take a few peaks down a notch and magic can really happen. Hope this helps!
(All pictures are from Audacity, printscreen'd and saved using Bitmap)