See the file to get an understanding of .idx contents.
Also to get raw PCM data you would need to decompress a chunk of data where the Index entry points (offset in .bag), some audio files are stored in mono and some are in stereo format (1 or 2 channels of audio)
Nox uses a modified IMA ADPCM algorithm, it differs in how stereo samples are interleaving. I has lost the working code for decompression but you can reconstruct in yourself after some experimenting.
Have a nice day.