Note 22:

Sotaque do norte

It seems the pronunciation of Portuguese in Portugal is quite uniform. There are few dialects and accents. Of course Mirando do Douro is a special case: it doesn’t belong to Portuguese, but to Asturian-Leonese, the national border doesn’t strictly follow the language borders here.
Alentejo allegedly has it own dialects, but I have nothing sensible to say about them.
Then there is the accent of the north of Portugal. Some traits (based on incomplete evidence):

Ão and final ‘-am’ tend to sound almost as ‘om’. The nothern accent shares this feature with Galician.
Written ou is really a diphthong in the north, something like [Ou] or [ou], unlike in the standard accent (Lisbon/Coimbra), where it is identical with the close /o/.
Written em, ‘en’ when followed by s, and final ‘ém’ do not sound as if written ãe, as is the case in Lisbon and an increasing number of other places. Instead it remains a nasalised monophthong. The northern accent shares this features with Brazilian and Azorian styles of pronunciation.
Likewise, written ei and éi do not become /3i/ as in Lisbon, but they remain [ei] and [Ei] respectively. The northern accent shares this features with Brazilian and Azorian styles of pronunciation.
‘Vêm’ and ‘têm’ etc. may not have the double diphthong found elsewhere, but instead a slightly longer monophthong than used in ‘vem’ and ‘tem’. I’ve only heard this once so far, in Fernando Lameirinhas’s translated version (1999 - Fadeando) of Jacques Brel’s ‘Le plat pays’.
Phoneme /3/ has a darker, perhaps also higher sound than in other parts of the country. Frequently heard in Fernando Lameirinhas’s songs.
There is something strange (I discovered in July 2003) about the /r/ in some northern accents. I’m not sure if it happens everywhere, perhaps only around Guimarães and Felgueiras? Also in Porto?
In 2006/2007 I discovered a lighter version of this phenomenon in the speech of radio presenter Helena Sampaio, especially in the commercials she does for this local radio station in Arcos de Valdevez.
I don’t know what it is phonetically, but my guess is some kind of retroflexion. It may happen also with other dental and apical sounds, like t, d, l, n, s? It is not the kind of retroflexion found in Indian languages, and in certain kinds of English r’s, where the whole tip of the tongue (some 1.5 or 2 cm) is curled upwards and backwards. Instead, perhaps it’s just the last 3 or 4 mm, that is curled up and back, while the tongue is still close to the tooth ridge.
The /b/ and the /v/, which are separate in the standard accent, tend to merge to a single phoneme with [b] and [B] realisations. The northern accent shares this with Galician and with Castilian (Spanish).
The northern accent distinguishes two kinds of [s], the apical s written s or ss, and the laminal [s] written c or ç.
(This is hearsay information only, I have never discerned this myself in real life.)
(For apical [s], cf. Castilian Spanish. For laminal [s], cf. Spanish and Galician [T].)