Building a pocket theremin
In modern audio circuits, no one worries about impedance matching. The usual technique is impedance bridging, which is in short, making the source impedances small compared to the input impedances. This minimizes the current the source must drive. It also makes parasitic capacitances in the cable and in the circuit color the sound less.

Consequently, you don't really need a resistor. There are two things that could be a problem, though:


This theremin could be much louder than a guitar
When the outputs become shorted, you could break whatever is driving them


On audio equipment, the outputs will become shorted. Sometimes this happens when the plug is being inserted into a jack, or someone might drop the cable on something metal. You should look at the datasheet for the driving element (the 555 IC) and see what what it says about problem #2. Some ICs say explicitly in the datasheet that they can deal with a short circuit on the output (for example, the TL072 op-amp). If they don't, they probably say what the maximum output current is. If the output is shorted to ground, a very large current could flow, defined by Ohm's law: \$I = E/R\$, where \$E\$ is the ouput voltage, and \$R\$ is the (small) resistance of the wires connecting the output jack. Find a value for \$R\$ such that the current can't be more than the rated maximum current for the 555, put it in series with the output, and you should be safe.

So what about problem #1? Well, maybe it's not a problem at all, in which case you are set. If not, a series resistor isn't going to work very well to reduce the voltage, and consequently the volume of your theremin. The reason is that your guitar amplifier is a high-impedance input. A high-impedance input means low-current. Consider Ohm's law: \$ E=IR \$. If \$I\$ is small, it doesn't much matter what \$R\$ is, \$E\$ will also be small. Here, \$E\$ represents the voltage, and thus the volume, lost over the resistor. Incidentally, this is precisely why impedance bridging is desirable.

The solution is a voltage divider. This will increase the output impedance of your theremin, but since the guitar amplifier presents an even higher impedance input, this shouldn't matter much. Also, this solves problem #2. If you want to adjust the volume, you can use an adjustable voltage divider, otherwise known as a potentiometer. The \$5k\Omega\$ pot already in the circuit should do nicely.

DC bias

There is one more potential issue: the output of this device will have a non-zero DC bias. That is, the average voltage it outputs will not be 0V. Since the 555 will either output 9V or 0V, you can see there's no way this can average 0V. Technically, this is a problem even if you don't connect it to a guitar amplifier: this DC bias serves to push the speaker's cone away from the center, on average, which maybe reduces the dynamic range and increases the distortion. But this isn't Stradivari violin, so probably no one cares.

The usual solution for audio circuits is capacitive coupling. Essentially, put a capacitor in series with the output. Your guitar amplifier likely has capacitive coupling on the inputs already, so it may not matter. And, if you have limited the maximum output current, it's unlikely you can do much damage even if this is not the case. So, maybe you want to keep things simple anyway. In any case, this would be best practice for a quality product.