So here it goes:
Lets say you have a bell on F2 and want to have a whistle on that key instead.
1. As mentioned above, you start operations mode programming, call up CV 300 and enter value 2 (for function key F2).
2. Exit operations mode programming with whatever key is used for that with your system.
3. The decoder is now in that special sound assignment mode. In this mode, the function keys are not used as normal but do exactly that which is shown in the manual. The speed regulator does not operate the motor either but determines the volume for the sound file on F2 (0 & 255 speed step = full volume, 1 - 254 = variable).
4. Pressing the F0 key plays back the sound currently assigned to F2, in this example the bell.
5. Press either the F4 or F5 key until you hear a whistle group (sounds in ZIMO decoders are grouped together with similar sounds). There maybe more than one group of whistles. A cuckoo jingle is played back when you reach the end of the group. Use the other key (F4/F5) to go the other way.
6. When you find the whistle group, use the F1/F2 keys to go for and back in that group and select the whistle you like. If there is more than one group of whistles use F4/F5 again to switch to the next/previous whistle group and then F1/F2 again to hear all the whistles in this group.
7. Once you found the whistle you want for F2 you can use the speed regulator to set the volume. Do this by pressing the F0 key repeatedly which now plays the whistle you just assigned for F2. While the whistle plays back turn/slide the speed regulator and listen to the volume change. Leave the speed regulator at the volume you like.
8. With F6/F7 you can program the decoder to playback the sound continuously (F6) and/or short (F7). These function keys must be ON at the time of exiting this mode to have the desired effect. The effect will only work if the sound sample has been recorded accordingly. If not, F6/F7 will have no effect but won't hurt anything either.
9. After selecting the sound sample of your choice (the last sample you selected with F1 - F5) and setting the volume if needed you can make the selection permanent by exiting this mode with the F8 key. A "confirmation jingle" is played at this point.
That's it.
The cab is now back to its normal self. That is you can drive the engine normal using the appropriate cab keys and speed regulator.
If you just want to remove a sound from a function key (not replacing it with another), you enter the same mode as above (at 1 & 2) but instead of selecting anything press F3. This will also exit the programming mode and at the same time erase the sound on F2 in this example.
You'll also end up without sound on F2 if you reach the end of a sound group with F1/F2 (when you hear the cuckoo jingle) and then exit with F8.
I hope this all makes sense. Once you've done it a few times it becomes child's play ;-).