To play a custom sound you must give it the FQFN (Fully Qualified File Name).
So yes, you must start with "/android_assets/www/", and it your file is in the
directory "sounds/", then it would be "/android_assets/www/sounds/".
Also, android only plays .ogg files (
http://en.wikipedia.org/wiki/Ogg).
There are many converters in Linux, and a few in Windows. I do not know
about Apple.
Let me know, if this helps.
Jesse