They (trees) shouldn't be cut down /?umbi:/ / ?uldntbi;/
NB: ? is written instead of the "sh" sound which I couldn't type.
My question is: since the answer is the first alternative, what should be the correct pronunciation and then transcription of the positive form? i.e,
Trees should be protected? What about must be and mustn't be? All these in spoken English ,of course.
Trying to discuss phonetics is challenging because of the challenge of the special symbols required. I teach pronunciation of American English and I am frustrated that, though there are general similarities, nearly every dictionary has its own way of representing various vowels and consonants of English. That being said, here is my take. I cannot hand write IPA (international phonetic alphabet), so I will improvise best I can.
should be /shood bee/ sounds like "good key"
shouldn't be Formal pronunciation is /shood'nt bee/ or more commonly, it is informal as /shood'n bee/ Sounding like "wooden bee".
This is complex to explain without the proper symbols, explanation of formal and informal pronunciations, and demonstration so you could hear it. With the second more common pronunciation in English, there are glottal stops applied to the 'd' and the 't', virtually eliminating them from being heard.
must be /must bee/ sounds like "dust key"
mustn't be Formal = /muh sunt bee/
Informal = /muh sun bee/ The 't' sounds essentially go away in informal pronunciation. Actually the 't' sound is half produced in what I call a 'stopped t' or glottal stop 't' sound.
For details, I would show you the various pronunciation notes on /t/ in recent Longman's American English dictionaries. I would also point you to the chapter in Ann Cook's "American Accent Training" on the "American T".