The /t/ in negative contractions in English can have three main realisations. In decreasing order of likelihood (all other things being equal):
- it can be a glottal stop
- it can be dropped altogether
- it can be a full [t]
It does not matter at all in negative contractions whether the following sound is a consonant or not in terms of dropping the /t/ altogether. It can easily be a vowel that follows.
So in terms of what native speakers actually do, by far the rarest realisation is with a canonical [t]. However, it is never wrong to use a normal [t] sound. Knowing that a [t] will usually not be present will greatly improve non-native speakers' listening skills though.
The other reason to be aware of the fact that there may not be a [t] present is that it enforces that fact that it is stress which is the most important factor for distinguishing negative contractions from normal auxiliaries. Negative contractions are stressed in English, whereas other things being equal most auxiliaries aren't when occurring in positive sentences. So when trying to distinguish between She can come and she can't come we will listen out for the following rhythms:
The first is what we expect from the positive polarity sentence. The second is the negative.
Assimlatory processes
The final [t] in negative contractions may be affected by the sounds following it.
For example, if the word following the contraction normally starts with [j], as in the first sound in you, then the /t/ and the /j/ may coalesce to form an new affricate sound, /tʃ/. This is the first sound that we hear in words like chair. So the string don't you may be realised as:
- 'doʊntʃu (Gen Am) "donchu"
- 'dəʊntʃu (British RP) "donchu"
Also if the following sound is not alveolar, both the /n/ and the /t/ may change their place of articulation according to the place of the following sound. So for example if the following sound is bilabial, the /nt/ cluster may be realised as /mp/. It is quite common to hear RP speakers saying I cam'p believe it, for example.
Given all your examples of supposedly rhyming words (they all rhyme for me a GenAmE speaker) having different possible pronunciations, both within a dictionary and between dictionaries, leads me to believe primarily not that there is some variation in pronunciation in the GenAmE population (though there very well may be variation (but consistent) between regional accents), but rather that these dictionaries are not scientifically consistent within themselves.
Either you have discovered an substantive inconsistency among these entries or these dictionary makers have performed research on all these pronunciation as part of their preparation but are not printing the outcomes/final results of the data leading them to make distinctions that are not meaningful for actual pronunciation or hearing.
You may want to construct a diplomatically worded letter to the editor in order to discover the reasoning for these pronunciations. I wonder if it would help to show one dictionary what the other dictionaries say?
Best Answer
In English, as far as I know, "rh" was never pronounced any differently from plain "r". This spelling, as @third-idiot wrote, is basically how the Ancient Greek "ῥ" was transliterated in Latin characters.
In Old Greek using polytonic orthography, an initial "ρ" was always written with a rough breathing, indicating the Greek /r/ (whatever its actual phonetic value) was probably aspirated or voiceless at the start of a word. Greek lost its aspirates quite early on though, and by the 4th century AD this rough breathing didn't mark anything anymore. Still, it kept on being written until the 1982 reform, which abolished the polytonic orthography along with the Puristic language and introduced the monotonic system and the Demotic language as official Modern Greek language (though it is still used by, for instance, the Greek Orthodox Church, which refused to acknowledge the reform).
People who made learned borrowings from Old Greek, like "hymn", "hypnosis" or "helium", transliterated the rough breathing on vowels as "h", and did that also for words where the rough breathing was on "ῥ", like "rhythm" or "rhapsody", even though they probably didn't pronounce that initial "r" as voiceless or aspirated themselves. It was just an orthographic convention.