Question: I'm a newbie when it comes to electronics, so please forgive any glaring mistakes!

I am trying to create a simple circuit that sends an audible frequency/tone to a line-in/mic input when the mercury switch is active. Could anyone help me to identify what component I would need in the circuit to generate and send the frequency to the microphone input?

Answer: 
You can use a [555](http://en.wikipedia.org/wiki/555_timer_IC) timer IC to create a tone generator

Here is a page that shows such a [circuit](http://circuit-diagram.hqew.net/Basic-Oscillator-%28Tone-Generator%29-At-1$2e8-KHz-$2dalarm-circuits_14287.html) but you can find many more from google.

![enter image description here](https://i.stack.imgur.com/1W83n.jpg)

You'll need to connect a pot (or a resistor divider) at the output to lower the output level to line in/mic levels.
The switch can be used in the power supply of the circuit.
Having the Audible Source, connected to the switch , and then to the mic input should do it.
Depending on number of wires in the Audio source(Say 2),you might need 2 switches to do this.