You don’t need the tach sensor anymore, just the two wires to the motor. You can plug your RS5 audio board into the motherboard. The speaker should be 8 ohm but any 8 ohm speaker is OK. If you want to use the speaker pictured just measure the resistance with an ohmmeter while its disconnected from the motherboard. It should read 6-8 ohms. 8 ohms is the nominal value.
Pete