Compiling a simple Objective-C program

I'm trying to create and compile a simple Objective-C program from the command line. Here is the code:

audio.h

#import <Foundation/Foundation.h>
#include <stdlib.h>
#import "WavReader.h"

int main (int argc, const char *argv[])
{   
    // LOG ARGUMENTS
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    NSLog (@"Running....");
    NSArray *args = [[NSProcessInfo processInfo] arguments];
    NSLog(@"Input File %@.", [args objectAtIndex:1]);
    NSLog(@"Output File Name %@.", [args objectAtIndex:2]);

    WavReader *wavReader = [[WavReader alloc] init];
    [wavReader testPrint];

    [pool drain];
    return 0;
}

WavReader.h

#import <Foundation/Foundation.h>

@interface WavReader : NSObject {

}

- (void) testPrint;

@end

WavReader.m

#import "WavReader.h"

@implementation WavReader

- (void) testPrint
{

    NSLog(@"Test print...");

}
@end

and then I try to compile it with

gcc -framework Foundation audio.m -o audio

and I get the following error:

Undefined symbols for architecture x86_64:
  "_OBJC_CLASS_$_WavReader", referenced from:
      objc-class-ref in ccs07gwo.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

How can I get this to compile?