This should be doable. To start, you can add your class as an observer of the MPMusicPlayerControllerNowPlayingItemDidChangeNotification
to be informed when the song in the iPod library changes. From there, you can tell the iPod library to pause, play your track, and then resume music playback afterwards.
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(someSel:) name:MPMusicPlayerControllerNowPlayingItemDidChangeNotification object:nil];
[[MPMusicPlayerController iPodMusicPlayer] pause];
// play your sound
[[MPMusicPlayerController iPodMusicPlayer] play];