Yup. In fact there's only one problem - the freezing is called by the problems with the sounds (when the scenario encounters an exception, it stops.) If you have a look at the java console, then chances are you'll see something like this: java.lang.IllegalArgumentException: Format of sound file not supported: URAN.wav at greenfoot.Greenfoot.playSound(Greenfoot.java:141) at Astroid.boom(Astroid.java:82) at Astroid.act(Astroid.java:29) at greenfoot.core.Simulation.runOneLoop(Simulation.java:200) at greenfoot.core.Simulation.run(Simulation.java:129) Greenfoot's sound support isn't great at the moment, and while improving the sound support is on trac, it's by no means a trivial task! The problem is while the API says it'll support WAV, it doesn't support all types of WAV, and that seems like it's the problem here. I can't remember the supported formats quickly, but try loading the sound into audacity, then exporting it as a wav file from there using the default settings - it's how I've saved many sounds in the past and I've never had a problem that way.

That's odd, does Audacity not handle it? It's coped with all the file formats I've used on it so far, including wavs that Greenfoot didn't like. Sounds like a stupid point, but you did select "export to wav" rather than saving it as an audacity project file? What error did you get? As for other ways, well you need to get it into a format that Greenfoot can play - I don't know of a program that'd do a better job than Audacity so I wouldn't know what to recommend if Audacity doesn't work. Unless you try making the sounds again (what did you make them in?) For now, you could just comment out the lines that play the sound and upload it - it won't have any sound but it shouldn't keep freezing like it does now.

Oh and just a comment on the gameplay - with or without sound it seems quite fast initially! Could you slow it down to start with and then perhaps speed it up gradually?

Well I'm not sure why your sounds aren't working, but have a look at freesound.org - you need to register, but when you do there's a pretty extensive collection of sound effects at your disposal.

It is rather difficult yes, and the problem with the sounds doesn't help because you can't use the laser without an exception being thrown.