Montage is not a sampler.
So we'll assume you created your audio with some unnamed recording device or program - and that this device isn't compatible with the Montage Waveform format. Correct? That's why they don't import properly.

You can take your audio samples open them in the Melas Motif Waveform Editor, you can have it automatically map them to Keybanks or you can manually set them up, thus creating a Motif XF Waveform which can then be exported to a file and loaded to your Montage. The Motif XS/XF file formats can be loaded by Montage (Voices, Arpeggios and Waveforms).

I did it like in this video:
https://www.youtube.com/watch?v=wo0lzByZWGs
,which works with just one sample which will by default be set to middle C as he says. but I read you need to have at least 6 samples to create a realistic instrument. so for this you need the Melas Motif Waveform Editor, whih I never heard of before? Ok, I ´ll try with that then, thanks.

Yes one .wav, but as you note, it takes more than one. A Waveform is a collection of samples mapped to the keyboard for Note region and Velocity Range. This cannot be done effectively on the Montage alone. If you already have the Melas Waveform Editor, you can use it now. But to buy the Motif Waveform Editor for Montage, well sure, it'll work, but I would email John and find out if he has a date for a Montage specific program.

I just found a post from july, don´t know if that´s still accurate

"I actually emailed John Melas asking him if the Motif version of the Waveform Editor will work with the new Montage. He replied that unfortunately No. but he plans to release a future version of the Waveform Editor that will support the Montage. So, I didn't bother buying the current Melas Waveform Editor, and decided to wait for the future version that will support the Montage. "

don´t have that editor, don´t really need to sample the melodica, I can just play it, it was just a test.

Using the Motif XF version of MWE covers most of the bases. You'll be able to export formats you need which comprise of multi-samples/keybanks due to Montage's compatibility with Motif XF formats.

My only difference with the strategy is in the "nickel and dime" category since it seems (note: speculation - the details of cost have not been clearly delineated for the future Montage-targeted tool - as of today users are directed towards an at-cost tool) at odds with the strategy taken for another "dropped vs. motif" feature - the sequencer - in which provides Yamaha provides, at zero cost, some level of basic functionality with a somewhat limited but capable tool (Cubase AI).

And, so, what have your learned from your test?

Hopefully, that the Montage is not a sampler.
It can load Waveforms (Motif XS/XF) Waveforms.
If you want to do this, hopefully you can be patient, and play the acoustic melodic until then. 🙂

And while the video shows how you would load a Waveform, if it is just a .wav it can only have a very limited use in Montage.

Are they claiming compatibility with Motif XS/XF and Montage?

Those tools will not work. I've referenced endless wav as one option to adjust loop points and root key (for a single sample) - but it does not generate a multisample (Yamaha targeted) waveform. The steps in your link include 3 pieces of software - the last step (using Sample Mapper) is where the tutorial creates a mutlisample in the SFZ format - which is really just going to be a compressed archive - this is not a Yamaha supported waveform format. I'm going by documentation and not 1st-hand usage of the Sample Mapper utility.

I don't think he was serious.

actually, there´s a whole article on user waveforms in french:
http://www.moessieurs.com/user-waveform_montage.html
he´s saying you need 4 samples per octave to make it realistic and you can map notes and velocity on the montage and he´s not talking about any external editor. he´s talking about keybanks without specifying what those are.
so yes, from reading that article I did assume I could sample an instrument and that it would be easy 😉
misunderstanding, I guess.

btw the melodica with just one sample on middle C doesn´t sound bad at all. it´s definitely usable!