As of 2010, Windows Media Player does not have the capability to edit audio files. To edit your audio files using Windows Media Player you need to download a plug-in that allows the trimming of Windows Media files.
Downloading a plug-in
As of November 2010, the only plug-in for Windows Media Player that allows you to edit the length of audio files available was the SolveigMM Windows Media Player Plug-in. It is a good idea to carry out a thorough Internet search to check for available alternative plug-ins before downloading the SolveigMM version.
Using the plug-in
Once the plug-in is installed it needs to be enabled. This can be done by clicking on Tools/Plug-ins/SolveigMM WMP Trimmer Plug-in or, if the main menu is not visible, by clicking on Now Playing/Plug-ins/SolveigMM WMP Trimmer Plug-in. Once enabled, it is possible to open audio files using the plug-in and trim them using the Trimmer tool bar. This is essentially a cut and paste process whereby it is possible to select the section of the file you want to cut out and save it as a separate file. This does not affect the original audio file.
The alternative to using a plug-in to edit files within Windows Media Player is to download or purchase a standalone audio editing program. You will find many freeware and private options available including Audacity, Goldwave, WavePad (which are free in some versions) or the Magix Music Maker.