Hi Tidus, welcome at HTG!
There are several ways to change icons, mostly mentioned in the link LH put up:)
But if you want to change the icon of the filetype, you need to change the settings in the registry if you want to have control over the specific icon.
Like the article said (and mind you - working with the registry is always a risk, so do make a good backup before you start and also export any keys in the registry you want to change.
If you work often directly with the registry I can highly recommend Registry Workshop (not free unfortunately, but absolutely worths it's money).
I'll take mp3 for an example:
First you look up .mp3 in HKEY_CLASSES_ROOT
Then on the right side after (Default) you see a name.
In my case 0000.mp3 (in your case probably something like 'audiofile')
If you browse downwards you'll find a key called 0000.mp3 (or 'audiofile').
A subkey has the value DefaultIcon. On the right you can change the value to any icon you like. And voila, you're done.
However - if .mp3 refers to 'audiofile', chances are other audioextensions refer to that too.
Like .wav and .mid. The result is that if you change the icon, all audiofiles will get the same new icon.
Because I like every audiofile to have it's own icon, I came up with a simple solution.
In the key .mp3 I changed 'audiofile' to 0000.mp3
Then I copied the key audiofile and named it 0000.mp3
Then I changed the icon.
And I did the same for .wav
and for .mid
So now every soundfile has it's own icon, but they still get opened by the same program as before.
BTW. I renamed all extensions I edited to 0000 + the extension, so I can easily find them again, when I need to.