1. Gardeners - The tree of life obviously, and from inside information I know that the two circles represent the shared heritage with the Soulons, which also appears in the Soulon symbol. The owl and wisdom that Kal pointed out is also interesting.
2. The Al - Reminds me of a hive, which the musics theme of communism confirms, otherwise I dont know. It also kinda reminds me of a nebula, and the lines are quite thin compared to other symbols, but whether or not that signifies something, I have no clue.
3. Ik - As everyone has said, Islam obviously, and middle east in general. But the triple crescent also looks like shockwaves originating from the star, which seems rather smaller in size comparatively, possibly representing the Ik perception that they have that powerful an effect regardless of whether or not they do.
4. Lantians - Asian origin, mecha china works too! The music, if anything, uses irony to highlight the Lantian aversion to any real combat.
5. Esani - Of course theres the sun, but thats too general. Theres the Asian origin, the black spot may be foreshadowing the impending doom regarding those ruins.
6. Vazan - Like Kal said, Diamond network! It fits in because it would mean both sets of related societies, Vazan-Kation, Gardener-Soulon, have a symbol for the shared past. It does remind me of a microchip, which is shown internally of the diamond, cyborgs!
7. Kations - The Diamond Network again. Um
Ill go with Milarquis suggestion of the Staff of Hermes for their high speed/dodge.
8. Soulon - Two circles horizontally arrange indicate a community, and these are bound by strong Soulon bonds. The kink means that this is also the symbol for magnetic iron, with all the connotations of that. The two cirles motif also appears for the gardeners to show a shared hertitage. The symbol is in triplicate becuase it looks cool.

I thought of chevrons and the military with the circles as planets initially also.
9. Akresian - Um, possibly because this symbol is quite different from the others that it symbolizes the physical differences of the Akresians compared to everybody else. Ill go with Ken and also say struggle.
10. Liberes - Pirates obviously! Chaos! Death they cause! The three skulls could be the three ship designs the Liberes are reliant on.
11. Priors - Black Hole! Nah
.both Ken and Kal could be right.
12. Aliens1 - Evil Reptilian Aliens!
13. Aliens2 - Sun worshiping primitive aliens! Reminded me of the Logitech corporate logo at first glance. That could also be a giant gear, some mechanical silicon based civilization, however, what the inner dot and circle would mean I dont know, especially with Through the Fire and the Flames (Good Song) as the theme.
14. Aliens3 - Vaguely familiar, probably because it reminds me of Galileos drawing of Saturn. It could be one star (system) that controls three others, similar to what Shadow said but applying to stars. But with the more peaceful theme compared to the last two
15. Aliens4 Well the bottom half of the symbol is the bottom half of the symbol for Earth in Stargate, the pyramid part. An Ending could either mean through destruction (Heze!), or finding Earth.
/end rambling/
