I will start with example:
I played the "Violin solo 1" voice and I wan't the sound to be straight without the vibrato movement.. So I tried to it by going to the LFO menu and set the wave speed slower but it doesn't effect.
Why is this?

Because the Voice you have selected (and the choice you have made in controlling it), is not using the LFO to synthesize the vibrato. What you are hearing is the real natural vibrato as played by the musician who was sampled. This makes for a much more realistic vibrato.

And what can I do to slow down the wave shape?

It is not an artificial wave shape being applied by an LFO. If that is what you would like: apply your own vibrato... you should use the MOXF's XA CONTROL to select the violin Element within this same Voice that is 'sans' vibrato (recorded without vibrato). Simply press the [AF2] (Assignable Function 2) button - its LED lights.

Just so you begin to learn how to explore these Voices on your own:

The VOICE you have selected "Violin Solo 1" is a solo violin Voice that is made up of 8 Elements. Each Element includes an Oscillator, with its own tuning and Pitch parameters, a Filter, a FEG, an Amplifier, an AEG, routing to Effects, it's own volume, EQ, note region, etc., etc., etc. when you initially call up an play the Voice you hear Element 1 - this is the main sound, if you really attack the upper range of the violin you will hear a pronounced "spiccato" bow stroke contributed by Element 8... But only on hard key strikes.

Element 1 is mapped from G2 and above - the natural range for a violin.
Element 2 is a composite string sound - a creation made by the programmers that fills out the notes that are out of the violin's range, namely notes F#2 and below... This composite is made from a viola, cello and contrabass as necessary. They do not leave notes below the violin's natural range blank, they use this "composite" Waveform to fill the region. Obviously a violin does not have the F# below middle "C" to sample! 🙂

Elements 1 and 2 only play when both [AF] buttons are off

Elements 3, 4, 5 and 6 only sound when [AF1] is On. Here you have a violin without the bow stroke, a composite wave, and an a Element that thickens the tone.

Element 7 is the violin without vibrato, you can play this Element only when [AF2] is On.
Element 8 is that velocity curve spiccato stroke... Set to play at all times, but scaled so it sounds in the upper reigns.

So to play the "Violin Solo 1" Voice without vibrato, simply press [AF2], now you can use the MW to control the LFO Pitch Modulation Depth, as you desire.

XA CONTROL is what gives a Voice several different articulations that you can manipulate in realtime as you perform. Found on the Element OSC - WAVE screen.
Press [EDIT]
Press [1] to select Element 1 parameters
Press [F1] OSC
Press [SF1] WAVE

While in Edit buttons 1-8 select Elements, the buttons 9-16 in the row directly below this row indicate which Elements 1-8 are active in this Voice.
The Assignable Function buttons are often used to change the way a Voice plays.

You should make it a point to explore the Controller assignments and the XA CONTROL settings when you call up a Voice... Usually there is much more than meets the eye within each Voice.

So what you say if I understand you is that I can't control the wave vibrato because originally they recorded it with vibrato?
I tried to play with the AF2 but it's change not just the vibrato.
When I play the "QUARTET" voice there is a lot of vibrato what make them less realistic.

What I said, applied to the Voice you mentioned originally. "Violin Solo 1"

I tried to play with the AF2 but it's change not just the vibrato.

If you play a violin range note (between the G2 and above... G2 is the G below middle and above) you are sounding just Element 1... If you press [AF2] it changes from the Element 1 Waveform to Element 7... which is the violin non-vibrato Waveform. It changes the entire Waveform, not just the vibrato. As I tried to explain in the previous post, Element 1 the vibrato was recorded because the original player used vibrato. The Element 7 is included when you would rather play without the recorded vibrato - it is sampled like most sample-based keyboards do it, with just the raw note, looped, without the natural vibrato of the player, it is programmed so that the MW applies both LFO-PM and LFO-FM to just Element 7 (pitch modulation is vibrato, filter modulation gives a slight timbre change when the wheel is raised) and if you explore you will see that only Element 7 is set respond to the MW:
Press [EDIT]
Press [COMMON EDIT]
Press [F4] CONTROL SET
press [SF1] Set1/2
Here you can see that MW (Source) is programmed to affect ELEMENT 7 (Element switch 7 appears). The DEPTH is the amount applied when move the Mod Wheel.

"Quartet" is a different Voice altogether - I did not give you an analysis of this Voice.

Try taking a look at the Elements within each Voice you decide you want to try to play. Learn what makes them work. On the "Violin Solo 1" specifically, you'll notice that Element 7 is made from a "Non-vibrato" Violin Waveform?
Press [EDIT]
Press [7]
Press [F1] OSC
Press [SF1] WAVE
Here you can see the name of the Waveform (Waveform name appears on the top line), and when it is set to sound (XA CONTROL) very important info.

Do you now know how to look inside a Voice? If you wish you can substitute the non-vibrato Waveform for whatever Element you don't like. It is a synthesizer, if you want to build a "better voice", do so!!! Build your "quartet" using the non-vibrato violin, let us know how you make out.

Try isolating ELEMENTS play each one alone so you know not only "what" it contributes, but WHEN it will contribute.
While in EDIT turn off all but Element 1 (represented by button #9 being lit... Turn OFF 10-16) here you are playing just Element 1, notice it does not cover the full keyboard. Notice it only sounds when both AF1 and AF2 are OFF.

It becomes just as important to know when an Element is NOT contributing to the sound as it is to know when it IS sounding.

In turn, isolate, and learn about each Element and what range it is set to play, under what conditions it is designed to sound... Then you are better equipped to create a Voice that includes the articulations YOU want to perform. Trust me, it is worth the time and effort to try this.

Then try substituting the "Violin Non-Vib" Wave Number for the violin Waveforms in the Voice "Quartet"