I'm really guessing, but I think the idea behind the ferrofluid is to more efficiently conduct heat away from the driver (as compared to air). Guessing the ferro part is to prevent the fluid from spattering all over creation.
But that is speculation, and I am not sure how the fluid gets cooled in turn.
The fluid would increase the resistance compared to air, but depending on it's viscosity, it might not change the sound too much.
This would be an argument for warming up your speakers if it did! Fluid viscosity is often temperature dependent!
When in doubt, look it up on Wikipedia
https://en.wikipedia.org/wiki/Ferrofluid#Loudspeakers
After a general explanation, there are shorter sections dealing with applications for ferrofluid:
Loudspeakers
Ferrofluids are commonly used in
loudspeakers to remove heat from the voice coil, and to passively
damp the movement of the cone. They reside in what would normally be the air gap around the voice coil, held in place by the speaker's magnet. Since ferrofluids are paramagnetic, they obey Curie's law and thus become less magnetic at higher temperatures. A strong magnet placed near the voice coil (which produces heat) will attract cold ferrofluid more than hot ferrofluid thus forcing the heated ferrofluid away from the electric voice coil and toward a
heat sink. This is a relatively efficient cooling method which requires no additional energy input.
Heat transfer
An external magnetic field imposed on a ferrofluid with varying susceptibility (e.g., because of a temperature gradient) results in a nonuniform magnetic body force, which leads to a form of
heat transfer called thermomagnetic convection. This form of heat transfer can be useful when conventional convection heat transfer is inadequate; e.g., in miniature microscale devices or under reduced gravity conditions.
Ferrofluids of suitable composition can exhibit extremely large enhancement in thermal conductivity (k; ~300% of the base fluid thermal conductivity). The large enhancement in k is due to the efficient transport of heat through percolating nanoparticle paths. Special magnetic nanofluids with tunable thermal conductivity to viscosity ratio can be used as multifunctional ‘smart materials’ that can remove heat and also arrest vibrations (damper). Such fluids may find applications in microfluidic devices and microelectromechanical systems.