Originally Posted by: Ross 
What protocols have you set on the ECoS for the Engine and the sound decoder?
Originally Posted by: PeFu 
I also believe this could be the cause. As mentioned in posting #2, ensure Mfx, DCC and analogue is disabled within LokProgrammer.
Originally Posted by: river6109 
... keep the motor connection and every other wire has to be unsoldered, install your fx sound decoder with the cube speaker
Boy am I glad that you folks have taken interest in helping me out! Thank you so much.
After much trial-and-error -- and with all the feedback that I've received from all of you -- I think that I've managed to solve this problem.
Now to address the various issues raised in this thread.
1. "Consisting" is not really the issue in this case since both decoders are in the same locomotive. Note: Ross has done an incredible job in his write-up on his web site -- amazing! I often refer to his web site and try to absorb his knowledge.
2. The LokSound 5 FX decoder was programmed with my LokProgrammer: Address 20 (MM protocol only) -- i.e., Mfx, DCC, Selectrix and analogue modes are all disabled.
3. As predicted, the problem was with protocols!
Background: I use the ECoS only as a "command station", i.e., to send multi-protocol digital signals to the layout. I use TC for control of the layout. My accessories are all DCC and my locomotives are a mix of DCC, Mfx, and MM. I don't have anything programmed into the ECoS, except locomotives. The default protocol in the ECoS for new locos is "DCC 28" and for new accessories it's "DCC" (see picture "ECoS Setup").

Because the V200 has an MM decoder, it does not automatically register in the ECoS. Therefore, I used the ECoS "new loco" setup menu and entered my loco. It came out as "Motorola 14", address 20 (see picture "ECoS - V200 Address 20 (MM)").

When the LokSound 5 FX decoder was entered into the ECoS, it automatically was assigned the DCC protocol because of my ECoS defaults (see background info above and picture "ECoS - V200 Address 21 (DCC)").

Once I corrected this problem, I had access to the FX decoder's function keys (see picture "ECoS - V200 Address 22 (MM)").
Problem solved!

Additional notes:
The ECoS allowed me to assign 8 functions under the V200 address 20 (MM). Because of this, I only needed one additional V200 entry in the ECoS to control function F9-F16. (see picture "ECoS - V200 Function Keys").

The addresses used are "20" for the original C-Sine motor decoder and "22" for the LokSound 5 FX decoder. I'm not 100% sure why that is, but maybe because MM decoders only had 4 functions, the ECoS uses two addresses for each set of 4 functions (i.e., 20 and 21).
Last question: Now I have two V200 locos showing up in TC. Is there a way to hide one of them? Can the FX decoder "loco" be suppressed or merged into one?
Thank you once again for all your help!
Andry