Interesting... I suspect that apps which output directly to OSS are fine, while apps which try to output to /dev/audio (converted by sadasupport to OSS API) have the faint volume problem... I suspect audioplay will have the volume issue too on that .au file, does it?
You can compile quite a few programs via SFE
which can allow us to verify this further, like xmms.
If this is the case, I don't know how it fix it - I'm a Linux user - maybe there's another mixer control program which needs to be activated? audioctl? You could also recompile stuff to use OSS directly (SFE helps here I guess).
[edit: P.S. Can you put the wav file somewhere I can get it? I want to examine it. ossplay should play most wav files well... ]