The decoders of early mfx locos are not locked against updates. You can upload new sound projects with an ESU lokprogrammer.

Later decoders (approx. 2006 and later) are locked. You need a new decoder to upload a new sound project. You may need new speakers, too, as there are different speaker impedances in use.

fx locos can be upgraded to sound decoders if they do not have the C Sine motor and if the space allows the speaker(s).

I never installed a sound decoder, therefore I can only provide some general information.
If the current decoder is plugged in, then you may be able to do the upgrade with little or even no soldering.

If you use ESU mfx (m4) LokSound decoders for upgrades, you can upload sound projects with the ESU programmer... Is it possibe to do it with an Ecos too?

As Tom says, the first M mfx sound loks had an "untampered" ESU LokSound decoder, but somehow they managed to lock them so they became less flexible...

M has introduced some new decoder kits, also with sound, but I don't know if you can upload sounds to them - if possible, it will probably be with CS2 with a very late hw/sw version...

If anyone knows more about the new M decoder sets, please make a noise about it...

Hi Pål,

as we discussed earlier this might depend upon the decoder HW&FW version.

The (quite extensive) documentation which comes with the set(s) actually refers to CS-II V2.0 and to even more information on the Märklin server which will be made available soon.

Hello Kevin,
I have early and the latest Marklin Loks with MFX decoders and sounds. There is probably not a lot to be gained just yet in upgrading the sounds of your early (2005) MFX Loks, you can only use sounds from the ESU menu anyway as the new Marklin ones may not necessarily be compatible, nobody knows yet as Marklin haven't made them available.
Another vital point is the early decoders do not have much memory space, so probably won't hold a newer sound project. I have upgraded or should I say one of the list members here in Auckland NZ has upgraded for me, several analogue and FX Loks. It also depends what type of decoder your present Lok has, ie is it a 21 pin decoder or a wired in type. If its a 21 pin variety one of the new Marklin ones or an ESU one of the 21 pin variety could be used. With ESU you really need an ESU Lok programmer to do the job succesfully, Dennis allways starts, decoderwise at least, by having the Lok programmer read the decoder for it's settings or CV's. Usually they will need to be programmed into the replacement decoder, this is absolutely vital with C Sine propulsion as the settings are different to the 5 star or non Sine Can motors which themselves are different.
I had a brand new M39800 MFX C Sine V200 go up in smoke on it's first run, two of the wires from the decoder board to the CSine driver board were jammed under the body mount and shorted out causing the complete burnout of the decoder. A small repair was also necessary to the Decoder board as an IC had been burnt thru. Dennis replaced the Decoder with the 21 pin ESU one and downloaded the V200 sound project from their site and put it on the decoder via the ESU Lokprogrammer and away it went, well not quite, The 21 pin decoder you buy from ESU has by default, the sound for a Klass 64 Steam Lok on it, the 64 of course is C Sine but anyway Dennis knew or obtained the C Sine CV's for the V200, adjusted the decoder accordingly, then it went and yes with a lot more sound functiuons than you get with a standard Marklin Issue C Sine V200. I have three of those now but I will not bother to upgrade the other two, because the MAIN sounds are really no different. Presently we are waiting on the new Marklin ones to upgrade a 37900 V290 I have, to sounds, double "A" light function (for shunting) and telex couplers. We are hoping the new Marklin ones will have an extra function output which will allow the Telex couplers to be switched independantly from one another. Dennis also has a CS2 which will enable the handling of the new Marklin sound project. Now sorry if thats a bit boring for some but, I just wanted you to know Kevin, that decoder upgrades involve more than just a physical swap. So to answer the fourth part or your first question it's fair to say it's not something a novice could do easily, but we all start with no knowledge
I decided to short circuit the learning process by getting somebody who allready had the knowledge and the equipment to do the job. See; http://www.thetraindoctor.co.nz/ for a few free ideas.
You also need to make a descision on a controller ie CS2, or ESU Lok programmer to work with everything else, when making a decision on which decoders to use for your upgrades. What you have now is probably a good starting point
Let us know how you get on, you will get helpful suggestions as you go where needed
BTW I have no financial or other interest in the Train Dr (Dennis) I'm just another satisfied customer.

Glen,Dennis,Pal,Tom and Juhan thank you so much for your words of advice, i will take in ,all what you say. It helps me very much and i will update you when i come round to doing the updates.

Thank you very much

Kevin