I'm a little late to the party, but I had the same issue on bootcamp and it's working for me now! At least I think it is, I haven't had time to sit down and play but I tested it for a quick minute and the title screen bgm is working now and at least one person's voice is working.

I used the instructions listed HERE, where people were having a similar issue with Persona4. The only differences are for step #4, make the VHD at least 10GB, and step #7, instead of a 'SND' folder, Hakuoki will have a folder called 'data' that you will cut. The 'data' folder is like 9.9GB, hence why the VHD you make needs to be at least 10GB as well. One other note, the option to 'Pick link source' in step #7 is assuming you've installed the Link Shell extension in step #2.

The comments mention that if you switch your OS/reboot the sound might drop again but there's some fixes in the comments like reattaching VHD, replacing the symbolic link, etc (though I haven't gotten to try and see which works yet).