There's no sampler in the S900, is there? Didn't think so, anyway...
The .wav playback is for tracks. Kind of redundant if it has mp3 (just use 256kbps or higher, and there's no way the electronics of the S900 can make the .wav sound better!), but there's no way to load a keyboard sample set in there.
Now the T2 and PA800/2X have samplers, so that is doable (and Roland have some scat voices in sampler format, but nothing that Yamaha read easily - or Korg, for that matter), but don't they have scat sounds already?
Of course, if you HAVE a Roland with scat voices, you could always prepare up an mp3 (or .wav) of a sequence, with the Roland scat voices in it, but you can't PLAY it...
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An arranger is just a tool. What matters is what you build with it..!