Capous,
 
What kind of audio are you working with?  Is it a soundtrack or is it narrative?
 
There's one thing I can think of to circumvent the issue you're tackling.  If all you're looking to do is add some background music to a presentation, you might want to try the following:
 
Depending on the kind of audio file you're working with, you can either embed the audio directly in Flash and publish that, or if you know how to use the Sound object in Flash, you can dynamically call and load the file at run time.
 
From there, you can modify the index.htm page that Presenter creates when publishing your PPT deck; because from that html file, you can easily add your swf which *should* allow you to play that background audio for the duration of your presentation.
 
The only wildcard in this possible solution is the kind of audio you want to add; namely, if you're looking to stream narrative (voice over) audio, then you're pretty much stuck with having to add that audio via the Import Audio or Edit Audio functions directly in PPT.
 
Rob