## following on from your code

## If the values are wacky try 'nn' instead of 'vv'
my( $leftVol, $rightVol ) = unpack 'vv', $vol;

my $leftPercent  = $leftVol  * 100 / 0xffff;
my $rightPercent = $rightVol * 100 / 0xffff;

## Numerically correct if not to a sound engineer
my $totalVolPercent = ( $leftPercent + $rightPercent ) / 2;