For an internship, I'm gonna have to develop a desktop application. The focus is creating a rich UI ( cool effects, sound etc ). Which tech should I go with ? - Flash ? ( in this case, shoud I go with a flex project ? AIR ? what is the gain between this and a simple raw flash project ) - C#/Silverlight ? Any other possibility ?
If experienced developers could give their advice, that would be great thanks.
Depends what programming languages are you proficient in. If you are .NET/C#/VB.NET guy go with SilverLight or WPF. If you know ActionScript, go with Adobe line of products.
The best technology to use is probably the one you're most familiar with, unless you mean you're going to be learning on the fly. But for the various options I'll take a stab.
Flash itself (the runtime) is a browser plugin, so that's not what you want, but if you build your application in Flash or Flex (the authoring tools), then you can publish it as an AIR application. AIR is also a runtime - basically it's Flash, plus a bunch of extra APIs for running outside the browser. The advantages to making an AIR app are:
The main disadvantage is incomplete access to native (i.e. OS) capabilities. The way a runtime allows you to run the same app on win/mac/linux is by restricting you from doing some of the things that you could do with a native application.
As for Flex, whether you use Flex or not is a separate question. Flex is a framework within Flash - an AIR app can be made using Flex or not using it. Basically, if you expect your app to have a lot of controls, slider bars, charts, accordion panels, etc., you might look into Flex. If you expect it to have a lot of animations and pictures and sounds, plain-old-Flash may be better.
Apart from Flash, the other main choice for a runtime is Silverlight. The advantages there are, to be honest, basically the same. You use .NET instead of ActionScript, and the tooling integration is with MS' Expression suite instead of Adobe's creative suite, but the development process and the results are quite similar. The choice between AIR and Silverlight is really a choice between Adobe tools and MS tools, and between two programming languages of entirely comparable power. Most everyone simply uses the one they're more familiar with.
Finally, the other set of options is not to use a runtime, and to simply compile an app from any language/development environment you like - C, C++, .NET, Python, anything. The advantage here is you get (in general) full access to OS capabilities. The disadvantages are, you (in general) need to do extra development to run on multiple platforms, and also I don't think you'll find a language/environment that lets you work with multimedia as easily as you can in AIR and Silverlight.
My preference goes to the (slightly) most opened universe. Assumed that you know how to write Object-oriented programs, Adobe is using Eclipse as a base for his tools, using plugins. Actionscript is just like Javascript syntax. You just have to learn the flex 4 libraries. Then, publish you app as an Air desktop application that can run everywhere. Really everywhere
The only thing to know about MS tools and languages and libraries is that they are very well designed and usable. and it runs on Win and mac also. and Only.
If the main request is to have a rich UI the simplest approach is to build a Flex application running under Adobe AIR platform. The answer to the questions Flash or Flex is very simple: if you are a designer go with Flash, if you are a developer stay away from Flash and use Flex. The Flex framework was created for developers, and is really hard to replicate all its features in Flash (think only what you have to do in Flash if you want to resize your app).
On the other hand if you are coming from a .NET background it will make more sense to stick with Silverlight/C#
I'm not sure if this helps but I'm developing in Flex and now I have started to learn Silverlight and I can say that developing in Flex Builder is much more faster then developing in Silverlight( Visual Studios ) . For me Flex Builder is much better because you use only one IDE to develop both back and front end. And that's when it comes to web development. If you want to do desktop applications then you do the same thing and a plus is that you get everything in only one file. You also have a small SQL database that comes with it.
But on the other hand when you develop in Silverlight you have to use several IDE's or tools outside Visual Studios to make awesome graphics. But if you want to have more time for design then I would suggest to use Flex Builder and not Silverlight.