Swift
Since neither answer is complete, and the solution is disseminated in the comments, let me offer a Swift alternative. and a step-by-step response.
The original Objective-C code is correct:
NSURL *audioFileURL = [[NSBundle mainBundle]
URLForResource:@"audio"
withExtension:@"m4a"];
Swift
let audioFileURL = NSBundle.mainBundle().URLForResource(
"audio",
withExtension: "m4a")
Target Membership
This was probably an oversight when originally adding the resource to the project. You must select adequate targets when adding these files:
Regardless of the language, ensure that your file is Project > Build Phases > Copy Bundle Resources. You need not to do that by hand. Instead, use the File Inspector. Select your resource (on the left panel) and verify it's target membership (on the right panel):