Lol,

LOL- At Random Sound.
LOL- At jumping (so pointless)
But fun and ehh, 6/10 and 3/5

simple

A good way to pass the time. Very good first, how ever it had no lives,no colour,no score and no levels. Good luck with the 2nd if your making one

Not bad actually

Very simple but it needs a few thing.

- Timer
- Lives(3)
- Levels
- Score

If you increased the speed after the player lasts X amount of minutes. You could also make objects that should be collected like lives or coins that could put up your score.

Addictive

Simple but enjoyable, i found it quite addictive, and if this really was your first flash then well done :)

Hey man...

Most of what I was thinking has been said already, except maybe shorten the levels a tiny bit. (as stated before, attention starts to ebb pretty quickly). Here's an idea, if you can pull it off... perhaps you could spice things up a bit by making the cube grow larger on occasion?
Well done for a first submission. Thankyou!