A transistor has a diode from base to emitter that draws a huge current if the current is not limited with a resistor in series with the base. The value of the resistor must be calculated with the peak voltage from the amplifier and the base current required. The huge current can destroy the transistor and/or destroy the amplifier.
The maximum allowed emitter-base reverse voltage is shown as 5V on the transistor's datasheet. A diode must be connected from base (cathode) to emitter (anode) to arrest the high reverse voltage from the amplifier.
The transistor, amplifier or LEDs will be destroyed if a brightness potentiometer is set wrong.
LEDs are all different. They should not be connected directly in paallel unless they are all measured and sorted so that they all have exactly the same forward voltage. A current-limiting resistor must be in series with the LEDs.