I don't really understand why one constant is unnamed and not the other.
Unlike `CENTER_PAN_GAIN`, `60.` and `15.05` are only used once, and they have explanatory comments, so using named constants seems a bit redundant. I don't mind naming them if you think it would be better.
Could we use that to make it generate output the way we like instead of trying to compensate its default implementation?
Looks like we couldn't, as the filter flags are hard-coded in `fluid_voice_initialize_rvoice`.