You’re probably searching the search index, which only covers files in your
personal document folders by default. In build 5472 you can click Start,
choose Search, click Advanced Search, and change the Location to include
whatever drive(s) you want to search. Then you can search from there. For
example if you want to search for all mp3 files that end in 320kbps search
for *320kbps.mp3. Or if you just want to find all mp3s you can search for
type:mp3.
If you add that location to your search index you can do that search right
from the Start menu or from the Search window without using Advanced Search
Options. It always defaults to searching the index because that’s the only
method that’s fast enough to give you keystroke-by-keystroke search results.