How's "m" in "bem" and similar words actually pronounced -- as [m] or more like [ng]?

Question

For instance, "bem", "sabem". On one hand, since these are "m", I'd pronounce them as [bem] and [sabem].

On the other hand, I've read an article of someone who's a poliglot and he says regarding switching from Spanish to Portuguese:

Nasalising every n/m you see at the end of syllables (not between vowels). The easiest way to get used to this at first is to imagine it was written as “ng” in English. So “bem” (well) would be pronounced as [beng] and “parabéns” (congratulations, and commonly “happy birthday”) as [pa-ra-beng-s].

From movies and audios it's not clear whether similar words are pronounced with [m] or [ng].

What's the correct way?

Answer 1

[This answer needs to be refined to include the items raised by Jacinto in comments.]

Regarding bem

From the two options you present, the [ng] one is the closest one.

But although it's a good and helpful approximation, it's not quite correct.

Listen directly to natives speaking it

You can listen to a few very good and distinct samples of diction in forvo:

https://forvo.com/word/bem/

NB: the one from "Bem1" is a Brazilian accent and not a Portuguese accent.

IPA notation

The most correct IPA notation for bem is probably /ˈbɐ̃j̃/.

I can't figure out how /'bẽ/ or /'bɘ̃/ are invalid; those were my initial guesses.
I was also inclined to add a velar nasal /ŋ/ at the end, but all online resources I found indicate that the nasalization with the /~/ is enough. 🤷

These are the different sources I found¹ for bem:

• /ˈbɐ̃j̃/ in Instituto Camões;
• /ˈbɐ̃j̃/ in Infopédia;
• /bˈɐ̃j/ in Portal da Língua Portuguesa

Unfortunately the online tool phoneme-synthesis only supports English, and isn't capable of processing /~/ (for /ɘ̃/ etc.), /ɘ/, or e; otherwise we could refine this.

Regarding -m endings

There are two groups of sounds for words ending in -m:

• -am;
• -em -im -om -um.

-am (/ɐ̃w/)

These are past-tense forms, for the 3rd person plural of the Perfect Past and the Pluperfect:

• Falaram;
• Riram;
• Comeram.

It has the same sound as -ão: /ɐ̃w/.

-em -im -om -um

These have a common ending to them, a nasalisation of the vowel.
This is represented with a nasalisation, which in IPA is done using a tilde on top of the previous vowel:

• Bem: /bẽ/;
• Sim: /sĩ/;
• Com: /cõ/;
• Pum: /pũ/.

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¹ Found in Online Portuguese dictionary with IPA transcriptions

Answer 2

Don't be fooled by the letters m or n. There is no consonant at all at end of the word bem, or for that matter, in any words ending in m (more generally, whenever an m is followed a b or p or is in final position or when an n is followed by the other consonants). What the m indicates is that the previous vowel should be nasalized.

In the Lisbon dialect, bem is pronounced the same way as mãe, that is, with the diphthong /ɐ̃j̃/, but in other regions this merger didn't happen and it is pronounced /ẽj̃/. This is distinct from the monophthong /ẽ/ in tempo. /ẽj̃/ does not exist at all in Lisbon.

There are reports of the nasal stop /ŋ/ being used in some dialects and in Brazil, but this is not part of standard European Portuguese.