Electronic – Switching a divided voltage with transistor or MOSFET

In short: you can't. The 0.6V threshold for a BJT is a consequence of the physics of silicon P-N junctions.

A germanium transistor would work, but you will have to mail-order it, and it will be expensive.

A rail-to-rail op-amp indeed may be an option.

However, another solution is to make the voltage of your audio signal higher, rather than making the transistor threshold lower. You could do this two ways:

Make the emitter voltage lower

^{simulate this circuit – Schematic created using CircuitLab}

Now, the audio signal is 0.6V higher than the emitter. Of course, you'd have to come up with a way to get a 0.6V power supply, and probably adjust it to get just the action you want. There's another way...

Add a DC bias to the signal

^{simulate this circuit}

Here you can adjust the pot to add some amount of DC bias to the signal to get the sensitivity you desire. The capacitor serves to isolate this DC from your audio source while allowing the AC signal to pass. This is called capacitive coupling.

R4 exists to limit the base current in case R1 is adjusted too far. There's no point in biasing the signal above 0.7V since that would mean the transistor is always on, so R4 also makes the useful adjustment range of R1 wider.

Also, notice in both cases I've added a resistor to the transistor base. You don't want to make this mistake.

This is not a particularly easy circuit to control through a safe galvanically-isolated connection. Normally the approach used in a production design would be to couple the microcontroller to the mains via an optocoupled zero-crossing detector and then trigger the triac through an opto-triac. The power would be a function of the delay between the zero crossing and the triac trigger.

If you really want to use that "light dimmer" circuit, it's possible with a light-dependent resistor (LDR) and a variable light source. LDRs may be hard to buy in some countries because they contain cadmium which is not RoHS compliant.

By the way, I'm not sure what you intend by that voltage source but it has a rather uncommon RMS voltage of 170V. If you intend to have 240VAC RMS @ 50Hz, you should use about 340V for the peak voltage (240\$\times\sqrt 2\$).