I have a Marklin Gauge 1 Crocodile 55683 that I control with an ECos 50210. Everything works perfectly. However the locomotive sound is VERY LOUD. I have tried to adjust the volume level lower, but I have not been successful.

Any help with this would be very welcome.

Normal way to adjust volume are via adress CV 63.
CV 63 are master volume.
If you use DCC protocol you can choice value between 0-255 which are maximum.
Adress CV 63 can be program via PoM on the main track.

Yes, I'm very satisfied with the new Crocodile. It is not up to the exacting standard of the Fine Art, Be 6/8 II, but the Marklin was half the cost and has much more functionality. The 55683 Ce 6/8 III runs very smoothly. Powerful motors and very heavy weight help. Now all I need to do is quiet the volume. Standing next to a running, full size Crocodile is not as loud.

Did you use correct locomotive adress before start change value?
The adress of the locomotive must first be located before you start change value in various CV adress.
If your loco still have adress 3 in DCC you must be sure that is correct.
If not then in case it´s waste of time by change value in any CV adress if the loco adress is not correct.

If you use DCC and the loco still have adress 3 you must use it.
Adress 68 are MM protocol.
M4 are just like Märklins mfx protocol as unique ID.
If you still cannot change master volume then something is wrong with the sound decoder.
I did had problem with ESU sound decoder.
When i did changed value at adress CV 63 i was force to write down to value 90 if there was different in volume sound.

You do realise that if a loco such as the one you're asking about has the ability to use 3 protocols (mfx, DCC, MM) that there is a priority order in which the controller will use them.

The priority order is mfx, DCC, MM.

If you are trying to program with DCC and the controller has mfx (m4) enabled then mfx is always used and you will never be able to program with DCC.

The fix is to disable in the Ecos those track output protocols that you are not trying to program with leaving the one you are trying to program with enabled.

I've also found it easier to program these Marklin decoders (including the Gauge 1 ones) with a CS2 or CS3 rather than the Ecos.

It doesn´t matter what protocol you have enabled because both M4 and DCC do have same adress CV 63 to adjust master volume.
If you have M4 and your loco stand on the programming track or PoM you can change value in same way like DCC.
The problem are to choice protocol in the decoder first to use by enable or disable it some protocols that you do not want to use.

I did wrote decoder.
Not the system.