Swahili pronunciation is mostly pretty straightforward; but, in a word like 'mpya', what is the correct pronunciation? I mean, is it something like /ˈm̩.pja/, with a syllabic /ˈm̩/ receiving stress, or more like /m̩pja/, one single syllable starting with a consonant cluster?
It's with a stressed syllabic "m". All nouns are more than one syllable, so this tells us that it can't be a non-syllabic "m" here. The only non-syllabic nasals before other consonants are in the prenasalised consonants, which are mb, nd, nj, ng, mv and nz so any other combinations, such as mp, mean that you know the nasal is syllabic.