Fmod driver is deprecated and will probably be removed in a future release. Same for DSound (https://github.com/ryzom/ryzomcore/issues/65)
We cannot support the FMod backend from the open source end, since the version of FMod is no longer available, and thus cannot be licensed anymore. It also misses some features.
DSound should be kept as reference implementation, even if incomplete, since it's what the sound artists originally used. Specifically the 'EAX' reverb effects are not yet fine tuned to sound the same across all drivers. No need to ship DSound to players, though, since it's not useful there.
(The EAX effects were never implemented in FMod at all, by the way, the setting does nothing there. They do work in XAudio2 and OpenAL, but don't match the reference exactly.)