A friend wants to know if replacing sound files with custom sounds is frowned upon

as the title says, a friend wants to replace Reaper's death blossom "die die die" and replace it with "CUT MY LIFE INTO PIECES" (I know, real original) He is too lazy to ask himself, so I just want to know if anyone knows of Blizzard's stance on replacing game files or if anyone has done something similar and if they were banned or not. Thank you in advance.