Usage: SET BEEP_ON_MSG [ALL|NONE|[-]<level> [-] <level> ...]
  This variable lets you specify certain types of messages
  which will cause an audible beep when you receive one.
  The possible message levels are:
    PUBLIC          Channel conversation only
    MSG             MSGs only
    NOTICE          NOTICEs only
    WALL            WALLs only
    WALLOP          WALLOPs only
    NOTE            NOTE only
    CRAP            Is not used.
  See SET LASTLOG_LEVEL for more on these levels