Most Instructables are designed by kids who know NOTHING about electronics.
This kid shows a 9V transformer and guesses wrongly that a 9V battery is the same. WRONG!
The 9VAC transformer has a peak voltage of 12.7V and when rectified and filtered produces 11.3VDC, not 9VDC from a little battery that drops to 6VDC during its life.
If you connect your 6V battery (it will be 4V when it gets old) to where the transformer connected then the full wave rectifier will reduce the voltage to only 4.6VDC with a new battery and only 2.6VDC when the battery is old. Will your computer speaker amplifier work from your battery that is only 2.6V to 4.6V?