There should be some MX voice packs that you can get from Yamaha Musicsoft, but you should be aware that any "new" voices which you can get for the MX are created using the sounds which are already in the MX.
As I understand it, MX voices are made up of voice "elements," or sound "elements," which are like individual samples that can be used as building blocks to create voices from. Elements can be layered together, similar to how two or more voices can be layered together; and each element can also be restricted to a specific range of notes, similar to how the keyboard can be "split" for playing two different voices. Each element can have various parameters adjusted, such as filter settings (e.g., cutoff and resonance) and envelope settings (e.g., attack and release), similar to how voices can be modified. That's my understanding, anyway; I've never delved deeply into the process. So an element is a lot like a voice, but it's more "primitive" in the sense that it's just a single sound sample and its parameters, whereas a voice is often crafted from two or more elements to achieve a greater richness and diversity of sound.
If you want to compare it to a synthesizer, a single element would be comparable to a single oscillator, whereas a synth patch frequently uses two or more oscillators to create sounds that a single oscillator cannot create all by itself, with each each oscillator having various parameters or controls that you can adjust.
You can do a great deal with these voice elements, but what you cannot do is load new samples or elements into the MX. There are mobile apps and computer soft synths that let you create even more sounds using the MX, but the actual sounds are generated by the soft synths, not by the MX itself, although the soft synth sounds can be mixed with the MX's sounds.
Yamaha Music Synthesizer MX series feature over 1000 Voices from the legendary MOTIF series plus deep computer and iOS integration in a compact, lightweight keyboard and a new blue color!
usa.yamaha.com
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