Ok, first I want to congratulate you for being the first person EVER to go into such depth, and have read so much before asking "Which language do I learn?". Really, my jaw dropped when when reading your post, I'm VERY impressed.
As for my creditials, I've been programming for about 6 years, in many different languages, such as C, C++, C#, Java, Perl, PHP, Python, VisualBasic, and some other web languages and markup languages. I've worked for several software and web development companies, such as the one I'm currently imployed with Key Net Solutions.
Now on to your questions. Let me take them in the order you asked them:
First Aim
Those files could actually have been written in many different languages. So long as your language can create binary files, it could potentially create one of those. Although, to see exactly how to do that, I'm not sure because I've never worked with sound files. Though I'm sure you could make one in several languages.
Q1.
C, C++, C#, Perl, ASM
I'm not going to go into too much detail yet about why, because that would make my post ungodly-long
Q2.
Personally, I'm a very avid C# fan, so I'd suggest that. But you might have some really good luck with C or C++ too. Each of these languages has their advantages, C# is by far the easiest to learn, C is the fastest language, and C++ is very low level if you want it to be, meaning you can get into the guts of the OS. Although, C# has that capability as well.
A) C# and C++
B) C# is. And C++ is "in theory".
c) C#
d) C#, Perl, C++, and C all have free compilers.
e) not really, if you study the language well enough, you could pretty much find any answer you want.
Second Aim
They were probably written in C or C++. So to write plug-ins you'd need to use those languages...though since I'm a C# fan, I'm going to tell you that you can make one just as easy with C#. And acutally, you could write plug-ins with C# as well.
Third Aim
That really depends on the software you're using. Because they could have been made in just about any language imaginable. Also, you can't really write a "plug-in" for them unless you have the actual source, which is hard to come by unless it's an open-source piece of sofware. So you're pretty much just going to have to write your own.
Fourth Aim
Again, these could have been written in many languages...but probably ASP/ASP.NET C++, C, Java, or MAYBE perl. I really don't know, but I'm sure you could find out fairly easily.
Sixth Aim (since there is no Fifth)
Again, any software that could make a desktop app could have made these. Personally though, I'd say they'd be easiest made in C# or perhaps Delphi. Though they could be made also in C, C++, or VisualBasic.
Seventh Aim
OS's are most likely written in ASM or C. I know that some versions of Linux were written in C. Though ASM is probably used a lot too. You "might" be able to get away with know C++, but in the long run, you need to know ASM and C.
I hope this post helped you. If you have any other questions let me know