animate volume (fadein/ out)

Authoring SMIL 2.0 and trying to use the animate tag to fade in and out 
the sound of a single rm audio file.
Is there a limitation to the audio file type we can use as source in 
order for the fade work properly?
I am experiencing re-buffering and therefor stuttering audio during the 
fades (Realplayer 10).
I have used the code exactly as described in the real production guide. 
Even their own files provided for the code example are stuttering.
Any known issues with that?
thank you.