The grapheme êm
is not always pronounced with this
double nasalised diphthong,
but only in some words, like
and other verbal forms derived from verbs "ter" and "vir".
(Are there any other examples? I am not aware of one. Suggestions welcome.)
On the other hand, cases where êm gets the normal single nasalised diphthong are extremely rare: the only ones I could find, using the word list in the tool Ergane, are: têmporas (temples) and êmbolo (piston).
In the song I took the sample of the word têm © from, is also the word demência. Here the êm simply denotes a nasalised /e~/, which gets a circumflex accent because otherwise the Stress rules would dictate that the stress be on the i.
The difference between
(absense and presence of the circumflex accent) becomes a difference between the acute
and the circumflex accent in verbs that have ter as their second element:
Confer mantém (he maintains) and
Other verbs with this spelling / sound behaviour are abster, ater, conter, deter,
entreter, obter, reter and suster.
These accents normally indicate tongue height of the vowel (acute: low á / mid-low é or ó; circumflex: mid-low â / mid-high ê or ô). But in this special case they distinguish the single diphthong (no accent or acute) from the double diphthong (circumflex).