Sound file decoding (GH:OT)

Hi,
I've been trying to decode an audio file format from GH:On Tour for a couple of days without great success.
So far we found out the codec was some sort of modified ADPCM (probably vox) and converting using SoX gave correct results but there still are volume flickering problems.
Here's what I use with sox:

Code:

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sox -t vox -r 19996 -N -V song.hwas song.wav highpass 10

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But after a couple of days of trying, it seems it might be better if someone who would know how to work his way in the arm9 file could look how the file is actually decoded by the game.
However, if you want to give a look at the .hwas file I uploaded one there: http://www.mediafire.com/?ajmybmdezj1
Thanks for your time