I assume you're trying to read dB SPL? That's been done, but there are many "complications". If you [u]search the forum[/u] you can find lots of discussion.
The "arduino volt" divider needs to drop the voltage by half, and the "audio volt" divider needs to create about a 25% drop.
Why? See [u]voltage divider[/u]. Two equal value resistors will cut the voltage in half. In most audio circuits the resistors can sum-up to about 10K.
The easiest way to "get started" is with a [u]microphone board[/u]. It's got a mic, preamp, and an "Arduino ready" biased output, and it's cheap. (That particular board doesn't have a gain control.)
I have a balanced audio connection via and LXR-3 port and I need to read the voltage/decibel level of the audio.
Is it line-level or a microphone?
Balanced connections usually need a differential amplifier. Microphones need a preamp and studio condenser microphones need 48V phantom power.
You can find examples of op-amp differential amplifiers on the Internet. If you have a line-level signal you can build one with a "gain" of 1 to convert the differential (balanced) signal to unbalanced.
Generally, you can ground one signal line and use the signal unbalanced but that's a bad idea if the signal is being used somewhere else and you want to keep it balanced. You also have to be careful and check the specs of whatever you're connecting because you're shorting one signal wire to ground.
In any case the Arduino can't read the negative half of the AC audio waveform (it can be damaged by negative voltages and you can damage/distort the audio signal). You can bias a regular line-level unbalanced audio signal with a voltage divider (2 equal-value resistors) to bias the input at 2.5V. That allows you to read the negative-half of the AC audio signal. Then, add a capacitor to "isolate" the bias from your audio circuit ([u]example[/u]). 10k resistors and a 1uF capacitor will work. (The audio comes-in through the capacitor and the resistor & capacitor junction goes to the Arduino input.)
With the input biased at 2.5V the ADC should read about 512 with silence, and you can subtract that out.