Paulorenato has built a very simple audio mixer for his YouTube karaoke system. The mixer simply takes the output of the TV that streaming YouTube video plus the Karaoke microphone output and combines them before sending the signal to the amplifier (aka “Receiver”) input.
The advantage of a fully passive circuit like this one is that it doesn’t require a dedicated power supply and doesn’t add much noise or distortion. On the other hand, it does attenuate the signal somewhat, though the amplifier volume control can easily compensate for this loss. The optional input switch disconnects the microphone when not in use to remove loading from the normal TV output signal path.