if you load the audio file with "snd_create" that's possible through snd_buffer afaik

if you don't want to load it (to save memory), you can read it from the file header (for wave files at least)

get the C# wrapper:
for A7.85.4 and A8.30.4, Version 2.3.9
at http://acknexwrapper2.matthias-auer.net/ or visit the thread