SSML Does not work for me

While not offering a full explanation of the SSML functions, you can get the effect you want ("C-T-R") by simply breaking up the letters in the way I just did:

C-T-R (include the hyphens)

That works for us when we are trying to spell an abbreviation/acronym/initialism.

Of course. Been doing that for years.

The promise of SSML was the ability to do these and many other marvelous things with markup.

The only tag that seems to work is <break> as is found in an example provided by Articulate staff. This one uses English (Australia) Standard voices

And I have to use English (US) voices which are only Neural. Hence only the “Neural and Standard” voice tags are viable.

Well, I have discovered the issue with the <sub> tag and likely a few others that I have tried and failed with. The "Help" that Articulate provides on their SSML support page is inadequate. The syntax is not really spelled out. When I see the syntax on the Amazon Polly site, i discover the problem.

Supported SSML tags - Amazon Polly

<sub>

This tag is supported by generative, long-form, neural, and standard TTS formats.

Use the <sub> tag with the alias attribute to substitute a different word (or pronunciation) for selected text such as an acronym or abbreviation.

This uses the syntax:

<sub alias="new word">abbreviation</sub>

In the following example, the name "Mercury" is substituted for the element's chemical symbol to make the audio content clearer.

<speak>My favorite chemical element is <sub alias="Mercury">Hg</sub>, because it looks so shiny.
</speak>

So, to use the sub tag to spell out CTR instead of having it shortcut to "center" I would need to add something like:

<sub alias="see tee are">CTR</sub>

Obviously, just writing C-T-R is much simpler, as Joe mentioned earlier.  At least now I know to check the Polly site for more complete syntax.

continuing with the Polly site for help: Supported SSML tags - Amazon Polly

I was finally able to get the <w> tag to work for noun versus verb type pronunciation.

<speak>I would like to <w role="amazon:VB"> present</w> you with this <w role="amazon:NN">present</w>.</speak>