ExtAudioFile can do a fantastic job. One of the reasons to accept some compressed formats, such as MP3 and convert to PCM, is that when playing audio, there is significantly less waiting time. For example, an MP3 sound effect can take about a second to start playback when the same sound in PCM could start almost immediately. I know, since I convert my sounds to PCM during startup and eliminate the delays that I suffered from.
Download iPhoneExtAudioFileConvertTest and you will find everything you need to make your own conversions