Many "audiophile" products make improvements to an aspect that doesn't effect the endresult that much. A SD Card that produces less digital noise (on its supply rails, or via its emitted electromagnetic fields) is - of course - an improvement by itself. Can it possibly affect the audio of a player? It could, but its not very likely that its really a significant effect. We shouldn't hear digital noise at all if the player has been designed carefully. Its of course possible to keep away digital noise from analog circuits, of course, but its not super-easy, I designed mixed-signal stuff myself and struggled with that, too. My Laptop (not a cheap one) for example doesn't filter it 100%, its outputs has clearly audible digital noise, and my middle-class MP3 Player as well. A Sony branded one, by the way
And I don't consider myself beeing an "audiophile" or heaving golden ears.
So what do I think here... I guess that the rest of the digital circuit, the processor, its memory and display probably plays a bigger role in producing possibly audible digital noise, but who knows? I think its at least possible to find measurable improvements in digital noise at the outputs of (some) MP3 players of an electrically more "silent" SD card.
Filtering something does never mean that something is totally "gone", its always just attenuation. So double filtering, or components that are more silent in the first place, might improve things. The only question is, if it makes sense. For example if the effect of the digital noise produced by the SD card already sits way below the noise floor of the DAC. But thats very tough to answer without taking real-world measurements. Why not do this? I suggest that for a video :-) Take a middle class MP3 Player and compare a super-cheap SD Card with the audiophile card. Playback a silent MP3 and a silent WAV (higher data rate = more memory accesses) and measure the outputs with a scope & spectrum analyzer. In my opinion thats the only way to really say: Ok, this doesnt do anything.