"The finale bell". Sounds like there's something on the master channel buffing things up a bit, still empty high ends, but not as empty. Likely because you are using a drum loop throughout mixed/mastered by someone else that it helps fill out frequencies a fair bit for you. Its also more layered a song. Song clips badly on the cymbal hit mid way through, likely other issues by instrument throughout, but not with their audio out signal. Still a bit low on the high end, and has lots of low freq room to work within.
"Aurora borealis preview". Nearly completely empty highs, soft lows. Tons of db in head room on the audio wave.
"Sunrise, sunset". Sounds incredibly over compressed/goodized/limited/multiple things. Clips alot, not much head room, still lacks instruments in a higher register, but the reverb/overtones of other instruments that occupy likely have been pumped up so much by the earlier mentioned effects that they add a grainy quality to the song. The bass isn't very tight, parts its too quiet/not present, other parts its nearly woofing if it weren't for a brickwall limiter you likely have on it. This is worse in mix quality then your newer stuff, because the newer emptier stuff can easily be buffed up without causing too much error, where as this for example has lost quality, and you can't mix/eq/master the mp3 back into a higher quality. You could go back and make changes to fix it though. Not like the song is dead forever.
Always better to be too quiet instead of too loud, because too quiet doesn't risk quality loss.
"Endless summer". Doesn't sound like the mix is clamped, more high end noises present, namely the snare. Low on the bass ends, mids are alright. This is a case of quiet instead of too loud and losing quality. Tons of head room.
Overall, sounds like you EQ out too much of instruments sounds by frequency when your songs aren't super dense with instruments to be able to afford that, and not have a full noise. You do have more volume for room, but I won't recommend remedying your emptiness through that. Its likely more sensitive an issue to tackle, and being quiet is better then being loud. Work on the frequency filling out, and not killing your instruments color.
You sound (to my ear anyways) to use a lot of the sytrus. Alot of instruments come out of that unmixed.
Send it to a mixer channel, compress it and adjust the gain to get it to a decent level. Open up the parametric EQ to checkout where its frequencies are and are not.
If it fills up too much bass, or too much high end, EQ it out. For bass if you have too much, you can do a bass roll off (where it is a gradual slide down on the left side) to soften it. If you have a very very thick bass sound, you may want to do a hard knee cut on it (looks more angular then a soft roll). If your bass sound is complimentary, just lower the signal, don't cut it out.
For high ends, you probably don't need to EQ anything out since you lack it generally. There is a lot of room in the high ends that it can be hard to fill out, so don't clamp those sounds down. If they are shrill, do a very very soft high end roll off (soft clip), or do specific frequency punch out if there is a bad resonance. For example a hihat that is super duper shrill at 13,000hz.
Don't EQ clamp your instruments frequencies down unless there is a clear problem. Don't eq/compress/limit/goodize/filter everything, this kills dynamics and often dampens the song. If it isn't ataking up all the mixing space, don't reduce it.
Use the monitoring tools available.
Also keep using the help thread.