The behavior of the function buttons / sound buttons on a cab are not defined by the DCC system. The decoder in the locomotive defines which sounds are controlled by which button. The DCC system only sends button commands to the locomotive. How the loco responds to the button being pressed is decided by the locomotive DCC decoder. To find out what button does what function, or triggers a certain sound on your locomotive, read the documentation that came with the loco, consult the company's website or contact them by phone.
The Function keys are the number keys on the keypad. The numbers on the display match the buttons on the cab.Example: 5 on the display means you pressed the 5 / F5 button on the keypad. Note: If your locomotive decoder is not programmed to respond to 5 / F5 then it will do nothing.
The Function keys that control sounds of a locomotive are based on the manufacturer of the Locomotive and / or Sound Decoder. This does not directly pertain to your NCE DCC System. The manufacturer of the sound decoders decide what sound goes to what button not NCE. Also note that most locomotive manufacturers do NOT make the decoders that are installed. Sound Decoders are a specialized item made by a small group of companies (Soundtraxx, TCS, ESU/Loksound, etc...). You will need to know who makes the decoder in your loco. The value of CV8 in the sound decoder tells you who makes the decoder. for more information go here. Consult the documentation that came with the locomotive for this information or the website of the manufacturer. Once you know which Function key activates or adjusts what sound on the locomotive, read this article for instructions on the Function keys F0-F28
To make changes to the sounds you will need to know the CVs you need to change, and what you need to change them to. This is based on the manufacturer of the locomotive sound decoder and not the system you use.
Once you know what CVs to change, and what to change them to, this article will tell you how to change a CV with your NCE system:
Soundtraxx - http://www.soundtraxx.com/manuals.php
ESU / Loksound - http://www.esu.eu/en/downloads/instruction-manuals/