The problem is that the apps in an iPad are isolated ('sandboxed') so that one app does not have access to another app's filespace. So music, for example, stored in the iPod application cannot be accessed directly the Keynote app. It's not technically impossible because Keynote could register with iOS (the iPad's operating system) its ability to handle music or audio files. Then, in the iPod app, it would be possible to use the iOS 'Open In' facility, which asks iOS if there's another app out there that can process that format of file. If iOS finds it has an app that's registered its capability to process audio files, then it will be offered as a potential 'destination' when using 'Open In'.
If you selected Keynote iOS would make a copy of the audio file and transfer it to Keynote's private file space, where it would be available. Maybe for a future release - we can only hope....
Maybe other Members know of a workaround - there is often a way - so hopefully you'll get some more feedback. Our Member twerp poet seems to know almost every workaround there is, and he's posting at the moment, so I'm sure he'll soon be here with some better advice than mine.
Tim