What exactly APE file is?How to open it,what window application associated to open the APE file?
APE is a Monkey's Audio file. Monkey's Audio is a TOP quality lossless audio compression codec . compare to MP3 ,the quality of MP3 is not near as good.APE audio it is lossless, there is no quality loss from the source audio once it is compressed. It achieves a compression rate of about 40%.
However,the only weak point is that the files are much larger than simple MP3 files which is why MP3 files would be more available online than APE files.And that's the reason why you seldom heard about the APE file.
How to play back APE files
How to play APE files,basically you can download a plugin and and play Monkey's Audio with Windows Media Player.Monkey's Audio is proprietary software and such you don't see support for APE files in many players that would would normally expect.
Monkey's Audio files use the filename extension .ape for audio, and .APL for track metadata.
Monkeys Audio APE Decoder Free Download

You can download the free version of Monkeys Audio APE Decoder here.

This plugin enables Pocket Player to play back Monkey's Audio (.APE) files.
Installation:

Copy the plugin (DLL) to your Pocket Player install folder, via the Explore function in ActiveSync. The install folder is "Program FilesConduitsPocket Player" by default.
Requirements:
* Pocket Player
Alternatively ,you also can download Windows Essentials Codec Pack to play APE file.You can downlad it for free at Cnet.

Features of Monkey's Audio:
If you have thinking to store a best quality audio file in compress mode,Monkey's Audio is a fast and easy way to compress digital music. Unlike traditional methods such as mp3, ogg, or lqt that permanently discard quality to save space, Monkey's Audio only makes perfect , bit-for-bit copies of your music.
- Efficient (fast and great compression): Monkey's Audio is highly optimized and highly efficient
- Perfect sound: absolutely no quality loss, meaning it sounds perfect and decompresses perfect (it's lossless!)
- Media Center, Winamp, and more support: supported by many popular players and rippers
- Easy: the Windows environment interface is both powerful and easy to use
- Free: Monkey's Audio is completely free!
- Error detection: Monkey's Audio incorporates redundant CRC's to ensure proper decompression of data (errors never go unnoticed)
- Tagging support: Monkey's Audio uses its own extremely flexible APE Tags so you can easily manage and catalogue your Monkey's Audio collection.
- External coder support: you can use Monkey's Audio as a front-end for all of your encoding needs
- Freely available source code, simple SDK and non-restrictive licensing - other developers can easily use Monkey's Audio in their own programs -- and there are no evil restrictive licensing agreements