I looked at the Fmod video on this , is it this complicated to add sound ?, is it possible to alias to the Citaton X sound by just renaming the files? 

Yes and No - You can copy any FMOD sound to any aircraft in XPlane by renaming the files to match the relevant aircraft acf, Whether it will work correctly is another issue because the FMOD will be configured to play certain sounds for different parts of the aircraft or its equipment, so yes you might be able to use the Citation X FMOD but when you run the model with this you may get error messages saying certain sound files are missing or not correct for that aircraft, it will not effect the overall way it works just need to know that will happen.

FMOD is a collection of sounds bit like a symphony of music with different notes and levels by different instruments tricky to set up as you have to understand the change in the sound depending on the position relative to the aircraft, number of engines types of equipment etc. 

I have done it quite a few times borrowing a good sound FMOD from one model and renaming it and putting it in the model with the bad sound. One of my favourite developers appears to either be deaf or happy with muted sounds so I have over time changed out or added in compatible FMODs that work for the aircraft. Getting an FMOD that is an appropriate match is not so easy (that is it is a sound package or FMOD that is for that engine type or type of aircraft) and at this stage of things with XPlane there are lots and lots of models still released with stock sounds and no FMOD at all. 

Putting together an FMOD is a big ask actually which is why there are so few released as alternatives for some models. I am still learning and working on it myself.

The FMOD in the Citation folder contains three  files ..... 1,Cessna_CitationX   2.   GUIDS  3 .  MasterBank.bank

do i rename one of these three ?                      BAE HS 125 Series 700 is the aircraft i want to add sound to

Copy the "fmod" folder from the donor aircraft and paste it into the receiver aircraft's folder, then rename the .snd file to match the name of the .acf file, e.g. "My_aircraft.acf" --> "My_aircraft.snd". That's it.

Edited September 25, 2025Sep 25 by Bjoern

Copy the FMOD folder. Rename the Cessna_CitationX to the same name as the acf file for the BAE HS 125 aircraft you have, for example if it is HS125.acf then change the snd file in the FMOD to HS125.snd  Thats it then move the whole folder into the model concerned. 

Edited September 25, 2025Sep 25 by coastaldriver

Thank you , I appreciate the help