This is how I played a beep sound effect in iOS 4:
SystemSoundId beepOnSoundId;
CFURLRef soundUrl = CFBundleCopyResourceURL(
CFBundleGet
One problem could be that in ios 5 when you call AudioServicesPlaySystemSound, it stop working if there is an active AVCaptureSession with an audio device active.
Also check the names of imports in your files, make sure the audiotoolbox import is in all your viewcontrollers. I also went to a guide and got this for you :) I'll leave the website's name at the bottom
Step 1: Import the AudioToolbox Framework Begin by importing the Audio Toolbox framework into your application in order to make the System Sound Services available to your code. To do so, select the “PhoneAppSkin” project in the Project Navigator pane in Xcode, then select “PhoneAppSkin” under “TARGETS”, and finally select the “Build Phases” tab. After doing so, your screen should look something like this:

Next, click the “Link Binary With Libraries” drop down, and click the “+” symbol to add a new framework to our project’s linking phase.

Finally, find the “AudioToolbox” framework in the popup window, and then click “Add”.

Next, open up the PhoneViewController.h file and add the line necessary to actually import the Audio Toolbox framework into your class:
#import
#import
The Audio Toolbox functions should now work
Website I got the images from: http://mobile.tutsplus.com/tutorials/iphone/ios-sdk_playing-systemsoundid/