1) The volume on your Macintosh Portable was low because the PRAM was
getting scrambled.
2) To clear PRAM on the Macintosh Portable, both batteries must be
removed for an undefined period of time -- probably more than 10
minutes. Otherwise, the 9-volt battery holds the values stored in PRAM.
The easiest way to clear this memory is, as you discovered, by keyboard
command.