first off, if the game MvC2 (Marvel vs Capcom 2) is for the Dreamcast system, the audio files will probably be on ADX format. you will need the tools to convert the ADX to WAV, check the bitrate, quality and properties of the WAV, redo the WAV for theway you like, and re-encode the WAV to ADX.
As for the SvC ( SNK vs Capcom ), the game doesn't exists for the Dreamcast. That game must be a ROM, and ROM musics are hard to change! I don't even know if it's possible for you to hack the ROM and change the musics of the game.
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