Second impression

[Takai]

So, I got it to the battle.

1. Interface. Take a second look at X-COM, Fallout Tactics, Silent Storm, and keep stuff grouped together in one section of the screen (lower part).

2. How about SS double-click system to move? The first would show the path and the cost in time units.

3. The sounds are REALLY bad. Can't you, for example, record stuff from movies? Like the shrieks from Aliens series or whatever?

4. It would be actually better if all soldiers started inside the dropship. UFO was excusable, because there were no interiors on the outside missions.

5. How about making Aliens bleed green? Or yellow? And scream when wounded too?

6. I'm not sure, am I supposed to actually click on reaction fire button to activate the mode?

7. I think peeking around to shoot/grenade would be pretty awesome. Enemy on reaction fire would catch an unit when half-exposed.

[Destructavator]

Two or three of these I can try to address...

3. The sounds are REALLY bad. Can't you, for example, record stuff from movies? Like the shrieks from Aliens series or whatever?

...And scream when wounded too?

The project has to use only either original content or content that is freely licensed under Creative Commons or GPL of certain types, because of copyright issues - Most commercial movies, films, and music are protected by copyright and therefore stealing content from them would infringe on the rights of those who created such content, leading to potential litigation.  This project is distributed freely, and one can not rightfully (legally) pass around something they don't have rights to.

If you are willing to create your own original works that you think would be better or would fill in the missing sounds, as far as I know anyone can offer a proposed contribution (as long as, again, they are original and not derived from copyrighted works).  If you have any programming experience you could also offer to help code some of the things on the TODO list (Sorry I don't have a link at this moment, one of the devs could help you with that.)

6. I'm not sure, am I supposed to actually click on reaction fire button to activate the mode?

I believe the button toggles that, yes, and if there are enough Time Units left you can also set reaction fire for multiple shots, if I'm not mistaken.

[Destructavator]

This might help you a little:

http://ufoai.ninex.info/wiki/index.php/Manual

I would especially look under "FAQ" and "Singleplayer," and although there are some incomplete sections, it's better than nothing.

EDIT: For contributing, you might find this useful:

http://ufoai.ninex.info/wiki/index.php/Contribute

(The devs could probably tell you more.)