There will be times when the audio files that you wish to use for sound effects are not in the format that you need. One prime example is a sound that you want to use with three-dimensional sound that is in a stereo format. Remember that all sounds used with 3D sound must be mono, since the sound system automatically performs the left/right mixing for the sounds. Numerous shareware and professional packages are available to solve this problem.
The best package that I have located for audio editing is Audio Edit by E-Soft (http://www.e-soft.co.uk). It is available in a wide range of languages and costs a modest $19. Audio Edit is able to work with both WAV and MP3 format sound files. In addition to its ability to convert a sound from stereo to mono, this package provides a wide range of effects that may be applied to any portion of a sound.