lost a few hours on this one, so want to share the trick :
- if one wants to play as song in standalone mode on the iPhone/iPad you need to have both the BIAB song and rendered audio file downloaded. The file failed to turn green because the song and the audio were not automatically merged on the iPad. so no audio.
- it turns out that the right rendering format to use is M4a and not AIFF nor WAV (I am working on an OSX version of BIAB). But that the app is case-sensitive with respect to the extension so needs to be m4a. just editing the extension in finder did the trick.

anyone a solution for transposed songs by the way ? I get he SGU uploaded in a non-transposed version