In order of appearance...
Dear Steve,
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I am actually enquiring on behalf of my modeller amigo who is fitting decoders to his locos.
Ah, "diagnostics by remote control", the eternal challenge... 
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He should have acquired the Tsunami 2 Alco diesel decoder, however, it may be the Tsunami 2 diesel decoder.
K, assuming he's started with
https://soundtraxx.com/content/Reference/Manuals/Tsunami2/ALCO%20Sound%20Selections.pdf
this goves us a "starting point" to work with. Reccomend a "CV8 = 8" Factory Reset so we can at-least get back to a "proven working as factory-intended" state, THEN we can start confidently working towards the desired result.
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He has inserted two of them thus far, one into each of his 80 class.
For those following along who are not from New South Wales, Australia, an "80 Class" is:

https://en.wikipedia.org/wiki/New_South_Wales_80_class_locomotive
https://vicsig.net/index.php?page=locomotives&class=80&orgstate=N&type=Diesel-Electric
CV123 = 6 would be my best-guess for the V12 251CE (century series) prime mover?
CV120 = 12, 13, 14, or 15 would be the best-guess for the Nathan P5 horn
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I have not yet been around to check out his progress regarding the function mapping attempts...He popped in to beg questions after suffering some quandaries with the function mapping.
Without wanting to be a smart-alek, it would appear the answer is "badly"... 
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I possess Decoder Pro, but do not possess a sprog nor any DCC enabled locos.
Pls don't mishear me, JMRI/DecoderPro is a fantastic tool, just like a cross-cut or chain saw is.
However, equally, the tool is only as "smart" as the Human wielding it, the Software programmer will not "make the decisions" for the Human, nor will it somehow intuit "what the Human actually wanted" when the Human chooses some options which aren't what the Human actually desires.
This is why I reccomend getting a very clear picture (write it down) of exactly which Physical Output/Sound effect is desired to be triggered by what "F-key" input,
and then, and only then, power up your DCC Programming tool of choice...
(I'd personally reccomend the PowerCab standalone, but then again I prefer to methodically "manually cross-cut saw" my way thru the process, rather than "throw a chain-saw at the problem and hope-for-the-best"...)
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Suffering the BEEP BEEP noise when pressing the horn function on his NCE Power Cab.
This symtom has me intrigued. Is the "beep"ing coming from:
- The sound decoder speaker (IE it's some form of "triggered sound")
- The decoder circuitboard (suggests some form of internal short-circuit?!?!)
- Some part of the loco mech? (get the shell off and listen close, high-frequency motor "buzz" could be described by some as a "beep", maybe screwed-up Motor Control settings?)
- From the PowerCab handset (weird, but maybe a Booster Overload condition?)
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From what I could see from Decoder Pro, it should be a simple method of selecting a function number and then selecting a desired sound option replacing the default sound.
Conceptually "cart before horse". Start by understanding that there are 2 (TWO) methods of re-mapping "F-key INput --> Physical/Sound OUTput".
Option 1 - "Legacy" mode.
Uses CVs 33 - 46. (NO "Indexed CV" schannigans required).
The paradigm here is explicitly each CV represents a specific "F-key",
and the value of the CV does a 1-> Many routing to a specific (limited) OUTput behaviour.
Option 2 - "Flex-Map" mode.
Uses CVs 257 - 321. To access these "higher than CV256" CVs directly/manually from the PowerCab,
you WILL first need to set
CV31 = 16
CV32 = 1
so the decoder works out "which CV 357" the User actually needs programmed.
This is the First Most-Common way contemporary "decoder programming" goes wrong,
and It is likely this chesnut is part of the "fault condition" equation in your mate's case.
NB Yes, using JMRI/DecoderPro should automatically set CV31/32 as required when programming CVs above 256, but this does NOT magically bypass the human's part in the "what we actually want to achieve" process... 
(also not the first time a bug in JMRI has meant the "Indexed CV" setting sequencing has lead to things "not quite working as-expected"... One trusts-in/abdicates-to an automated-tool at their own risk... pass the chainsaw...)
How does the above related to programming in DecoderPro?
1 - For TSU2 family decoders, use the "Function Map" tab,
AVOID using the "Legacy Function Map" tab!
2 - As you go down the "Extended function mapping" column,
using the drop-down menus to select the desired "F-key" number,
make sure to keep going all the way to the end, and set any "Duplicate" F-key entries to "DISable".
EG Lets say you want to move the "Grade Crossing" horn to "F5".
This WILL create issues because "F5" is already triggering "RPM+" by default.
Ergo, if you are committed to Mapping "F5" --> "Grade Crossing"
then make sure to REMOVE "F5" from the "RPM+" setting...
(set it to "Disable", or remap it to some-other F-key).
This is the Second Most Common way "function mapping goes wrong",
and is likely due to the Human "quitting once they find what they are looking for",
and failing to complete the process, confirming that there are no "latent/leftover cross-mapped F-key references".
Hint: JMRI does NOT "auto-clear" any cross-mapping of Given F-key --> TWO-or-MORE "Output/behaviours/Sounds"!
Rather, JMRI assumes that the User may Actually Want to have a given single "F-key" trigger more-than-one "Output" thing, even if that is NOT what the Human actually wants... :-( 
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But, regarding the Bell function which he desires to remove, inserting another sound effect does not seem to work.
See above:
(assuming we are starting from a "CV8 = 8" Factory-reset condition)
- SET CV31 = 16
- SET CV32 = 1
- SET CV298 = 255
(DISAbles the Bell, hitting the default "F1" F-key will do NOTHING.
This is a "sanity test" point in the process...)
Then
- determine the CV (257-321) which relates to the Sound effect you want triggered by the "F1" key
(See Tech Reference PDF Page 119 for "which CV relates to what Physical/Sound/Behaviour Output" table)
- SET that CV = "1" (value "1" --> "F1 key", see Tech Reference Page 118)
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But, I was hoping that there would be a simple explanation for the problem encountered.
Do you mean:
- "Explanation" of WHY the fault condition occured in the first-place
OR
- "Solution" (Get from Broken --> Fixed) to just-make-it-work ASAP
?
The "Explanation" ("Why is it so?") likely lies somewhere in the combination of:
- "Failed to set the Index CVs 31/32 appropriately"
- "Mixed Legacy and Flex-Map mapping modes"
- "Double-mapping of given F-key --> multiple Sound behaviours"
along with a dose of "Human trusted/powered-up an Automated Tool,
before fully Affirming and Documenting the Actual desired Result"...
...but One could get lost "down the diagnostic rabbit hole" for days if not-careful,
esp when diagnosing-from-afar, with incomplete/inaccurate diagnostic info...
Alternagively, the fastest-path-to-"Solution" lies in:
- Reset to Factory (CV8 = 8) and confirm nothing is terminally broken
(IE the decoder does what he factory says it does out-of-box, there are no wiring issues, etc)
- Confirm exactly the desired "F-key --> Physical/Sound" mapping,
INC clearing out or "remapping somewhere else" any unwanted behaviours
(Don't just stop once "F0 thru F9" is conceptually mapped,
There may be other "now cross-mapped" behaviours which can come back to bite!)
- Programming Slowly and Methodically in consequence.
(PowerCab + Tech Reference Manual,
or JMRI + Tech Reference Manual,
either "set of tools" will work, it's your choice...
)
Honestly hope this helps,
Happy Modelling,
Aim to Improve,
Prof Klyzlr