This is *probably* not a bug: the voice file you are using needs to come from the exact same build of Rockbox as the actual program, otherwise some things may play the wrong sample. I can't think of an easy way to test just the battery case, but it's likely that if that sample is wrong then so will others be: just turn on full voicing for menus and you will probably find that some menu options are voiced incorrectly as well.
We don't currently have any mechanism for Rockbox to let you know if you are using the wrong voice file, unfortunately.