Learn English – Word stress: Sorry to keep you waiting

Here is an approximation to what SPE (The Sound Pattern of English) predicts for the stress contour. The Nuclear Stress rule applies to the constituents in the syntactic structure to make the last primary stressed syllable have a primary stress, and all other stresses in the same constituent are weakened by one degree. I've bracketed the constituents that the stress rule is applying to. I've assumed that "it" and "the" are exempt from stress.

First, the words are stressed:

[1I] [1put] it [1on] the [1table]

Next, the prepositional phrase is stressed:

[1I] [1put] it [2on the 1table]

Then the verb phrase is stressed:

[1I] [2put it 3on the 1table]

Finally, the whole sentence is stressed:

[2I 3put it 4on the 1table]

The stress might be adjusted by what has been called a rhythm rule to make stronger and weaker stresses alternate -- here, "on" might get stronger stress than "put".

Of course, if the context requires some emphasis or contrast, stress is modified accordingly.

Maybe.

We judge the stress by pitch, vowel reduction, and perhaps in other ways. In your example, judging by pitch, I think I would say the first two syllables of "Alexandra" out of context with higher pitch on the first syllable than on the second, but in your example sentence, "Hey Alexandra!", I find it most natural to give those syllables the same low pitch. So, then judging from this, and using pitch to estimate the stress, yes, non-primary stress sometimes goes away in the context of a larger sentence.

However, the vowel of the first syllable of "Alexandra" does not reduce, whether or not it's in a sentence. (Other examples might work differently.) So judging from the absence of vowel reduction, no, I don't think non-primary stress does disappear in context of a larger construction.

Loudness might be another way we can judge stress, but I don't think I can do this, personally.