i know i can use a msgeq7, but i do not need to.
You some circuitry to protect the Arduino from the negative voltage-swing, and from any signals above +5V. The diagram on this page
with two diodes and a resistor is a good way to do it. I'd recommend a resistor value of 1K to 5k ohms, instead of 100 Ohms if you have line-level signals (or a headphone signal).* 100 Ohms is fine if you are always going to connect a headphone output.
i just want the whole cube to blink like one LED would
To make an LED blink with sound, here's how I do it:
I take a "reading" once per second and I create a 20-second moving average (See the Smoothing Example
and Blink Without Delay Example
for ideas on how to do this without slowing-down your main loop.)
I read the input signal every time through the main loop, and whenever the signal is louder than average, the LED comes on, and whenever it's lower than average the LED is off. That gives lots of "LED action" (on half the time, and off half the time), and it automatically adjusts to the average volume.
In my application, I also automatically switch between the 1.1 and 5V ADC reference, depending on the signal level.
* The diodes are protecting the Arduino from the audio signal, and the resistor is protecting your audio device/signal from the diodes.