You need a flag to remember that you've already sounded the alarm.

Code:
AlarmDone VAR BIT

IF Temperature > TempUp THEN
  PortA.2 = 0
  PortA.3 = 1
  IF AlarmDone = 0 THEN          ' Only activate alarm if not already activated.
    SOUND PortA.7, [107, 50]
    AlarmDone = 1                   ' Alarm has been activated.
  ENDIF
ELSE
  AlarmDone = 0                   ' Temperature is not over the limit, rearm the alarm.
ENDIF