Idea for a PC bios beep buzzer
Motherboard speaker connections allow directly use of 8 ohm speaker. That's how they were originally designed in the 80's and they still are compatible.
There is no need for an amplifier, and in fact the motherboard speaker output pin is usually open-collector output so direct connection to amplifier input would most likely not work, it requires at least some biasing network to convert the open-collector output to voltage output.