Whew! I found the problem.
I had an extra copy of the .h file outside the CocosDenshion folder.

I was editing the copy inside the folder but the one outside the folder was being compiled.

Seems almost like an xcode bug. The file that is being edited should have priority over any other versions of the file and if there is a conflict the compiler should alert you.

I can't see anything wrong. The only thing I can think is you have a typo somewhere but if it works then that can't be the case. Are you using CocosDenshion from a static library? If so maybe try explicitly cleaning then building the static library.

I've made sure to clean the build each time I've run it.
Here is the header "SimpleAudioEngine.h" (without the comments) :

#import "CocosDenshion.h"
#import "CDAudioManager.h"

@interface SimpleAudioEngine : NSObject {
BOOL muted_;
}
@property (readwrite) BOOL muted;
@property (readwrite) float backgroundMusicVolume;
@property (readwrite) float effectsVolume;
+ (SimpleAudioEngine*) sharedEngine;
-(void) playBackgroundMusic:(NSString*) filePath;
-(void) playBackgroundMusic:(NSString*) filePath loop:(BOOL) loop;
-(void) stopBackgroundMusic;
-(void) pauseBackgroundMusic;
-(void) rewindBackgroundMusic;
(BOOL) isBackgroundMusicPlaying;

-(void) audioResumed;

-(ALuint) playEffect:(NSString*) filePath;
-(ALuint) playEffect:(NSString*) filePath pitch:(Float32) pitch pan:(Float32) pan gain:(Float32) gain;
-(void) preloadEffect:(NSString*) filePath;
-(void) unloadEffect:(NSString*) filePath;

@end

That message should just be a warning, it means that the method is not declared. Can you post the actual header file you are using? Also make sure you have cleaned your build.

I've added a method to the SimpleAudioEngine so that I can continue playing sound after playing a movie. It works but I always get:

'SimpleAudioEngine' may not respond to '-audioResumed'

Here's what I've added:

SimpleAudioEngine.h
-(void) audioResumed;

SimpleAudioEngine.m
-(void) audioResumed {
[am audioSessionResumed];
}

I call it with:

[[SimpleAudioEngine sharedEngine] audioResumed];

Does anyone see what I'm doing wrong to get the error message?