From Halopedia, the Halo wiki

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Sound files on Halopedia generally use the Vorbis audio format, and video files use the Theora format, both contained in an Ogg file. These are analogous to other formats used to play digital audio and video such as MP3 and MPEG. However, Microsoft Windows, iOS, and Mac OS X do not support these formats by default, and require additional software to play them.

For more, see Wikipedia's media help.