ADC on an audio signal

Good afternoon Ladies and Gents,

I am trying to perform an ADC on an audio signal to around 20kHz and I'm taking 512 samples at a time. I then want to perform an FFT using the CMSIS library with this signal/sample to give me 64 frequency bins. The value from each of these bins will then dictate the colour of the led (there are 128 leds, 2 per frequency bin) in a later part of the project (just so you know), its sort of a spectrum analyser. 

Currently I believe my ADC is working in some form but it's doing dome strange things, meaning I've set something up thats not right or forgotten to. I'm using a pot form 0v to 3.3v, not an audio signal (the audio signal will be 3V pk-pk biased around 1.5V), currently, so I can monitor the change I'm putting into the system. 

I'm tracking the data being put into the arrays; audio_samples[ ] and adc_sample[ ], using the graph function so I can see all my samples
- Why are my values are all over the place? 
- How come adc_sample isn't exactly the same as audio_samples? Am i not just replicating the array outside of the interrupt?
- What would be the best sampling frequency to run at? 

Here's my graph: 

Here's my code; 

/*
 * main.c
 */
#include <stdint.h>
#include <stdbool.h>
#include "inc/hw_memmap.h"
#include "inc/hw_types.h"
#include "inc/hw_ints.h"
#include "driverlib/debug.h"
#include "driverlib/sysctl.h"
#include "driverlib/adc.h" //for the adc peripherals
#include "driverlib/gpio.h"
#include "driverlib/timer.h"
#include "driverlib/interrupt.h"

//#include "arm_math.h"

#define NUM_SAMPLES 512 //number of samples taken by the ADC


//declare ADC intrupt
extern void ADCIntHandler(void);

uint32_t g_ui32SysClock;
uint32_t audio_samples[NUM_SAMPLES];

int main(void)
 {
	volatile uint32_t ui32Loop;

	//set syst clk frequency
    g_ui32SysClock = SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ   |
                                             SYSCTL_OSC_MAIN |
                                             SYSCTL_USE_PLL  |
											 SYSCTL_CFG_VCO_480), 120000000);

	/*// set up onboard LED on PN0
	SysCtlPeripheralEnable(SYSCTL_PERIPH_GPION);
	while(!SysCtlPeripheralReady(SYSCTL_PERIPH_GPION)){}
	GPIOPinTypeGPIOOutput(GPIO_PORTN_BASE, GPIO_PIN_0); */

	//set up pin PE0 for ADC input
	SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOE);
	while(!SysCtlPeripheralReady(SYSCTL_PERIPH_GPION)){}
	GPIOPinTypeADC(GPIO_PORTE_BASE, GPIO_PIN_0);

	//set up the ADC
	SysCtlPeripheralEnable(SYSCTL_PERIPH_ADC0); //enable the adc
	ADCClockConfigSet(ADC0_BASE, ADC_CLOCK_SRC_PLL | ADC_CLOCK_RATE_FULL, 1 );
	ADCSequenceDisable(ADC0_BASE, 3);
	ADCSequenceConfigure(ADC0_BASE, 3, ADC_TRIGGER_TIMER, 0);
	ADCSequenceStepConfigure(ADC0_BASE,3,0,ADC_CTL_CH3|ADC_CTL_IE|ADC_CTL_END);
	ADCSequenceEnable(ADC0_BASE, 3);
	ADCIntClear(ADC0_BASE, 3);

	//variable for adc value
	//uint32_t Value;

	//setup timer
	SysCtlPeripheralEnable(SYSCTL_PERIPH_TIMER0);
	TimerConfigure(TIMER0_BASE,TIMER_CFG_PERIODIC);
	TimerLoadSet(TIMER0_BASE,TIMER_A, g_ui32SysClock); //calculate this to be the right freq for audio sampling and fft
	TimerControlTrigger(TIMER0_BASE,TIMER_A,true);
	TimerEnable(TIMER0_BASE,TIMER_A);

	//set interupts
	IntRegister(INT_ADC0SS3,ADCIntHandler);
	ADCIntEnable(ADC0_BASE,3);
	IntEnable(INT_ADC0SS3);
	IntMasterEnable();

	while(1)
	{
	}
}

void ADCIntHandler(void) {

	uint32_t adc_sample[NUM_SAMPLES]; //dont really need this; just use the global variable
	uint32_t adc_sample_count;

	for(adc_sample_count=0; adc_sample_count<NUM_SAMPLES; adc_sample_count++){ //writes the sample value to array
		ADCSequenceDataGet(ADC0_BASE, 3, &adc_sample[adc_sample_count]);
		audio_samples[adc_sample_count] = adc_sample[adc_sample_count];
	}
	ADCIntClear(ADC0_BASE,3);
}

Sorry if these are basic questions, I'm new to microcontrollers and the launchpad, any help you could give me would be appreciated (dumbing it down might me help too)

Will:) x