Adding audio-in port to plasma ball with built-in Microphone
My guess is the microphone is acting as a resistor divider for a PWM modulation controlling the voltage... It may not be straight forward to replace a passive microphone with an active audio input.

Your best bet is to diagram the circuit, and understand how the microphone is being used. If it is just varying the voltage to a pin on a PWM driver, you could scale the audio signal, and do the same thing. Preferably through an audio transformer (they are small) so you have some level of safety and can avoid ground loops etc.