Game Design Contest Prototype

Allright, I promised to submit anything new, didn't I?

Before I burst into endless blabbering about anything and nothing here are the controls:
Use the arrow keys.
Quite easy.

So, this is the prototype of a game that I might submit to a Game Design Competition (if I can meet the deadline - there are only 3 weeks left...).
Fine, now why am I submitting this?
Actually I was thinking about submitting this to the scraps, but I'd be really happy about some feedback!
The competitions theme is "ball physics"...right, the theme is a bit hidden in this prototype (I obviously won't have too good chances for receiving the "best use of the theme"-prize, but anyway). I've originally been toying around with a completely different idea which was 100% ball physics, but it turned out to be too hard to realize in Actionscript 2 (and I'm not familiar enough with AS 3 yet :/).

The current working title is "Asteroid Mining Company Pilot" (catchy, ain't it?).
I'm not too sure about the gameplay yet. The basic idea is that you fly this "Asteroids"-like spaceship (with the twist that it got some kinda physical behaviour). The goal would be to deliver chunks of minerals (depicted by the two circles that are attached to the ship in this prototype) from the deep inside of an asteroid to it's surface.
Now, the problem is that (as it should be a good game) there needs to be some replay value.
Just flying through a long tunnel over and over again get's pretty boring after a few times, doesn't it?
So, I think there need to be some obstacles...
I already thought about some types of obstacles, but they won't work if the ships flies as fast as it does at the moment. If I reduce it's speed it'll destroy the "holy shit I'm flying damn fast through a tunnel with really solid looking walls that might crush me anytime"-effect...meh.

So, what can I do?
Should I reduce the speed in favor of adding some obstacles?
Or are there any obstacles I could add without reducing the speed that wouldn't make the game unplayable hard?
Any ideas?

Obviously I need some good gameplay since the graphics will stay as minialist as they are now (since I'm not good at creating graphics, plus I kinda like the style). There will be some sorts of eye candy being added.
Another problem are sounds. A good game needs sounds, but I don't have any.
Does anyone know good sound resources that offer free to use sounds?

Edit: What would you prefer?
- one continuous, huge random generated level that get's harder and harder the further you come; this would easily allow to create a highscore (which is quite nice consindering the replay value)
or
- a bunch of smaller, handmade levels that increase in difficulty; this would maybe allow cleverer level design, but on the other hand once you've played a level you'll probably not play it again

_{Woah, I guess no one reads thus far...but because I learned it like this at school, here's a short summary: Do you have any ideas for obstacles/gameplay elements? Any comments on the prototype? I'd love to hear!}