DirectShow comes with everything you need to play MPEG1 files.
To play MPEG2 files, you must have MPEG2-compatible audio and video decoders. Due to patents covering this technology, DirectShow does not come with such decoders. For the same reason LEAD does not offer complete built-in support for playing MPEG2 files. LEAD does provide the LEAD MPEG2 Video Decoder (2.0), which is a separate decoder the can be used to play the video portion. The audio in most MPEG2 files uses MPEG1 audio compression, and DirectShow provides a decoder for it. Consequently, nothing else is needed to hear the sound in most MPEG2 files. Some MPEG2 files however, use other types of audio compression such as AAC or AC3. To hear the audio in these files, you must install a compatible decoder. LEAD provides the LEAD AAC Decoder and it does not provide an AC3 or LPCM decoder. It will, however automatically use such a decoder if available.