What do you mean by the “sound” of an Ondes?
Do you mean the waveforms? If so, all of them are fairly easy to recreate, save perhaps for the octiviant. Onde (sine), Gambe (square), petit gambe (filtered square), Nasillard (pulse), Souffle (noise), and Creux (basically a triangle/sine with slight square wave distortion) are all, more or less, available on your average synthesizer.
The more unique diffuseurs (Palme and Métallique) would be hard to reproduce, but the Principal is a more usual speaker, and you could replace the Résonance with some well-set spring reverb.
But the sound of the isn’t just the waveforms and the diffuseurs. A huge part of it comes from its unique method of control - the ruban (ring), the touche d’intensité (button), and the clavier (a keyboard which can move from side to side to create vibrato). That is why something like the Soniccouture Ondes, which reproduces the “sounds” aspects of the Martenot, sounds pretty lackluster compared to the real thing.
Some clever ADSR can reproduce the the touche to a certain degree. Other options like mapping joystick controllers to master volume can be even better if used well, but those are less common. A volume pedal is certainly an option, and the Ondes actually does have pedal control for volume in passages which require both hands (for the keyboard). Volume pedals will lose you the extremely fast “attack” and “decay” of which the touche is capable, but since the Ondes is better known for what it can do with a slower attack, that ability of the touche might not be important to many for a reproduction.
While standard mod-wheel+LFO vibrato will sound pretty bad for a Martenot reproduction, mapping aftertouch to pitch (subtly) can allow for more natural sounding vibrato. This vibrato will be around only a single pitch at a time, which can work to reproduce the clavier, but it still does not approach the most famous part of the Martenot - the ruban.
The ruban allows one to play any pitch along its range, allowing not only for real, player controlled glissando between notes, but also for microtonal playing. While the latter may be less important to the average musician, the former is probably the most distinctive part of the Martenot’s sound.
Now, almost all synthesizers have a built in means of creating glissando – glide. This feature is capable of some unique and interesting sounds, but it has a character very much its own. Simply put, I’ve yet to hear glide sound anything like an Ondes Martenot, and the Soniccouture Ondes is a testament to that. With the waveforms and the diffuseurs perfectly duplicated, it still fails to sound much like the original instrument. The glissando created with the ruban is controlled entirely by the player. Its “envelope,” so to speak, is user generated, and it’s unique every time. Glide simply sounds robotic by comparison. Not a bad thing in absolute terms – glide definitely has its uses – but not Ondes either.
So while sampling and synthesis are able to perfectly recreate the sounds of the Ondes Martenot, the “sound” of the instrument remains unique - to ribbon controllers at least.
Of course, you could always try a MIDI ribbon controller, such as those by Doepfer or Eowave! It won’t be perfect, and the possibilities will be limited, but it’s sure nonetheless to make any synth sound a lot the a Martenot!