I just did a quick check and it turns out filenames on Android can only contain ASCII characters, as all resources will be stored in the assets directory. (Explained a bit here.)
Maybe you could use the unicode identifier as filename instead of the actual character?
For example, unicode character ‘ཀ’ has decimal value 3904 (I used http://unicodelookup.com/ to find that out). So your file code be named 3904.wav.
I think this would be a reasonable workaround.
Hope this helps,