MX seems to have some provisions for expansions. See:

https://usa.yamaha.com/products/music_production/synthesizers/mx_bk_bu/sound_library.html#product-tabs
and
https://www.yamahasynth.com/mx-category/b-o-m-b-2-the-best-of-motif-bank-for-mx49-mx61-and-mx88

... there's more out there.

You can create new sounds with preset waveforms using external editors (JM Tools, for example).

Remote Editor
The MX Remote Editor allows you to assign the MX's knobs to control the VSTi functions of your choice. You can load factory templates or design your own, so I created new templates for the G-Media M-Tron Pro and Arturia's SEM V. The first did not work correctly; anti-clockwise movements affected the selected parameters as expected, but clockwise movements did not. The second worked perfectly. Bugs notwithstanding, this could be a valuable addition, especially if you like to tweak parameters while playing synths and, in particular, soft synths.

https://usa.yamaha.com/products/music_production/synthesizers/mx_bk_bu/downloads.html

MX49/MX61 Remote Tools V1.0.6 for Mac
MX49/MX61 Remote Tools V1.0.6 for Windows

The MX49/MX61 Remote Tools V1.0.6 consists of the following software components.
- MX49/MX61 Remote Editor V1.0.6
- MX49/MX61 Extension V1.0.6

The MX49 is an entry-level synth — it is the least expensive (full sized key) synthesizer in the Yamaha line-up. Based on the bread-and-butter sounds found in the Motif-series it is designed to be 16-part multi-timbral for working with a sequencer/DAW... or standing alone as a keyboard.

I’m not sure why Jason couldn’t program the Remote Editor to control his external synths (but if you still have issues you should post a separate question). Not sure why the Remote Editor is even in this conversation (I guess the word “Editor”... ). The MX can act as a remote control surface for computer DAWs and its knobs/buttons can be used for this purpose (This has nothing to do with the MX’s own sound designing capability).

If you are looking for a synth to do “sound design” you’d do better looking at a mini-key refaceCS (virtual analog) 8 note poly or the mini-key refaceDX (FM) 8 note poly.... each features a pure synth engine. And their mini-size should not deter you, they are fully MIDI capable, and takes nothing away from their ability to “synthesize”... sound design.

If you are looking for the least expensive (full sized key) sound designing synth in the Yamaha line-up you want to look at the new MODX6 (128 note stereo polyphony AWM2 + 64 note polyphony FM-X).

If you are looking for a full sized key, 88 note weighted action workstation synth, with free Editor and Remote Control Surface capability you may still be able to find a MOXF8 at a great price...

I’m not sure why Jason

You mean Ricardo? I've expressed zero issues with MX.

I entered the remote editor into the conversation because the OP was expressing issues finding VST features. If this matches his request for "VST editor" or not is unknown - but it's what MX has in the way of VST helper software.

I’m not sure why Jason

You mean Ricardo? I've expressed zero issues with MX.

Actually Gordon Reid. His complaints appear in the excerpt from Sound On Sound included in your first post.

The first did not work correctly; anti-clockwise movements affected the selected parameters as expected, but clockwise movements did not. The second worked perfectly. Bugs notwithstanding, this could be a valuable addition, especially if you like to tweak parameters while playing synths and, in particular, soft synths.

This had been very helpful. Although I am still considering returning it for a used modx6..

But in the meantime, I'm trying to use the keyboard with FL studio and I'm trying to have access to several sounds at the same time. .any help with this?

ok - I wasn't trying to emphasize bugs here. Just that there's VST-related downloads to explore. I believe MX is in the 2nd generation and the review seems like 1st gen MX. Things may very well be fixed by now.

This had been very helpful. Although I am still considering returning it for a used modx6..

But in the meantime, I'm trying to use the keyboard with FL studio and I'm trying to have access to several sounds at the same time. .any help with this?

What do you mean by “at the same time”...

This could mean playing several Parts layered... or it could mean simply using multiple Parts, one per Track, but building an entire ensemble sound.

The MX is always in multi-timbral mode. Always.
Part 1 is channel 1, Part 2 is channel 2, and so on Part 16 is channel 16.

See this post: https://www.yamahasynth.com/ask-a-question/how-to-output-the-16-part-multi-timbrel#reply-97104

The MX is, as has already been said, an entry-level keyboard aimed at live performance, and as such is is great at that job. It has loads of readily available sounds, and uses Performances which allow you to set up a group of sounds together for a specific set list, for instance, with very easy-to-set-up splits and layers, and a very powerful arpeggiator and rythm/drum section. For a lot of people, that is all they want.

However, and it is a big however, there is a huge amount more lying just below the surface. Yes, the keyboard is awkward to programme just using the tiny LCD screen, but there are a number of third party programmes available. The main one is the John Melas set which has a library programme allowing you to manage a large number of performances, and even re-arrange the performance themselves. You also get a Performance editor which allows you to programme all the settings involved in a Performance, including the assignable control knobs, and the common Chorus and Reverb settings.All the setting you can adjust are presented to you on the computer screen, so they are all visible and accessible at once, without having to go through page after page looking for the bit you want to change. Finally you get a voice Editor which opens the synth right up. You will find there isn't just one oscillator/sound source. There are actually 8 elements making a single voice up, with each element being a synth in its own right with its own oscillator/sound source, Amplitude envelope, Filter envelope. LFOs, pitch, keyboard range and then what are called insert effects. These are mainly chorus and reverb, with w few others such as Amplifier models, for example. With 8 of these elements availabe, the possibilities are enormous, but you are likely to spend a lot of time in programming!

I am absolutely bowled over by the keyboard - what I would be like if I could afford a more expensive keyboard I have no idea. I had 30 years out of playing music because of RSI, but have recovered sufficiently to get back on the ivories, and the difference in what is available and the price you have to pay has blown my mind!

I would recommend you have a look at the third party editors - they will cost, but the price is comparitively low, and what they give you is well worth the investment!

I'm trying to use the keyboard with FL studio and I'm trying to have access to several sounds at the same time. .any help with this?

Are you trying to have an FL Studio project access several MX sounds at the same time? Or are you trying to have the MX access (play) several FL Studio hosted sounds at the same time?