You could create two NSMutableArrays and empty them when you hit record.  You would also need an NSTimer and an int.  So in header:
NSTimer *recordTimer;
NSTimer *playTimer;
int incrementation;
NSMutableArray *timeHit;
NSMutableArray *noteHit;
Include in your header all of the voids and IBActions and such below.
Make your sound buttons all have different, unique tags.
and then in your main file:
-(void)viewDidLoad {
    timeHit = [[NSMutableArray alloc] init];
    noteHit = [[NSMutableArray alloc] init];
}
-(IBAction)record {
    recordTimer = [NSTimer scheduledTimerWithTimeInterval:0.03 target:self selector:@selector(timerSelector) userInfo:nil repeats:YES];
    [timeHit removeAllObjects];
    [noteHit removeAllObjects];
    incrementation = 0;
}
-(void)timerSelector {
    incrementation += 1;
}
-(IBAction)hitSoundButton:(id)sender {
    int note = [sender tag];
    int time = incrementation;
    [timeHit addObject:[NSNumber numberWithInt:time]];
    [noteHit addObject:[NSNumber numberWithInt:note]];
    [self playNote:note];
}
-(IBAction)stop {
    if ([recordTimer isRunning]) {
        [recordTimer invalidate];
    } else if ([playTimer isRunning]) {
        [playTimer invalidate];
    }
}
-(IBAction)playSounds {
    playTimer = [NSTimer scheduledTimerWithTimeInterval:0.03 target:self selector:@selector(playback) userInfo:nil repeats:YES];
    incrementation = 0;
}
-(void)playback {
    incrementation += 1;
    if ([timeHit containsObject:[NSNumber numberWithInt:incrementation]]) {
        int index = [timeHit indexOfObject:[NSNumber numberWithInt:incrementation]];
        int note = [[noteHit objectAtIndex:index] intValue];
        [self playNote:note];
    }
}
-(void)playNote:(int)note {
    //These notes would correspond to the tags of the buttons they are played by.
    if (note == 1) {
        //Play your first note
    } else if (note == 2) {
        //Play second note
    } else if (note == 3) {
       //And so on
    } else if (note == 4) {
            //etc.
    }
}
With a little bit of fiddling (I doubt this code is perfect), you might get this to work.  Like you will probably want the play/record buttons to be disabled once you hit one of them.  Good luck!