1. Each LED needs about 1.6V (for red, othe colors may need more) so with 4 in series you need at least 6.4V supply before there is any chance of them lighting up. I suggest 100 Ohms series resistor is also used to limit the current.
2. As the output impedance of your phone/PC is probably only a few Ohms, I suggest you use say 22 Ohms in the base.
3. It isn't essential but it is advisable, use a 1N4001 or equivalent with cathode end toward the transistor base pin.
4. Is the crux of the problem, the circuit is clearly designed by someone with almost no knowledge of electronics. It relies on the peaks of the audio waveform being able to drive the transistor into conduction, that means there has to be more than about 0.7V of audio (VERY loud in an earpiece!) before anything can happen. Voltages lower than that will not have any effect at all. With a suitable amplifier, rectifier and voltage offset you can make the LEDs start to light with a quiet sound and get progressively brighter as the volume increases. I guess that's what you really want.
Brian.