This paper describes one aspect of an ongoing work to incorporate pronunciation variability in the Portuguese (PORT) speech system. This work focuses on the linguistic rules to improve the grapheme-(multi)phone transcription algorithm that will be implemented. Portuguese ‘Beira Interior’ regional speech (PORT-BI REG) is considered to be in the realm of coarticulation (post-lexical) phenomena. A set of linguistic rules for most of the common vowel transformation in an utterance (vocalic segments at both the left and right edges of the word) is presented. The analysis focuses on the distinctive features that originate vowel sound challenges in connected speech. The results are interesting from the point of view of setting up models to reconstruct a grapheme-phone transcription algorithm for Portuguese multi-pronunciation speech systems. We propose that the linguistic documentation of Portuguese minority speech can be an optimal start for Portuguese speech system development process, too.