Simplify ShowSound() in FileMonitor.cpp.

Now if more sound types are found, they just need to simply be added to
the unordered set.

- Also changed ShowSound() to IsSoundFile()
- Fixed IsSoundFile’s definition in FileMonitor.h. This whole time it
has been defined as a void method, when in reality it was a bool
function.

- Changed the FileMonitor’s string parameters to be constant references.
Add two other formats to the list of extensions in IsSoundFile

.adx is used in various games, so this should definitely be here.
Merge pull request #131 from lioncash/file-monitor-simplifications

Simplify ShowSound() in FileMonitor.cpp.