Which language / Framework/ Server OS to be choosen

I agree with everything Ray_Paseur wrote, with the possible exception of using phpMyAdmin.  But I suppose that boils down to your tolerance for waiting...

A few other notes:

1. The size of the web site or the database behind it is the main concern, not the media used.  You will, however, need to consider your available bandwidth in the context of page requests.  Multi-media files generally take up the lion's share of any web page, so plan accordingly.

2. The speed of the database is most affected by its construction.  Regardless of which platform you choose, proper normalization and optimization is absolutely critical.  Normalization helps you to easily build complicated queries that "make sense" to the database engine.  Optimization helps the engine use the resource it has at its command.  

3. Support for flash is a client-side concern, and not something you can help.  Luckily, HTML5 introduces the video element, which can remove your dependency on flash altogether.  Also remember that flash does not do too well with search engines.

4. The great thing about the internet is that it is very much platform agnostic.  The platform you choose has very little impact on the platforms you can support.  To support mobile platforms, you need only use a custom style sheet, and proper, easily manipulated structure in your HTML.  

On the server side, I've worked with Linux and Windows, Apache and IIS, MySQL and SQL Server, and PHP, ASP, and (some) ASP.NET.  By far, I recommend the LAMP platform.  While it is not too hot on the GUI for managing things, the transparency of the services and the ability to atomically manipulate how they work win the day.  IME, migration is also much simpler.