We know that the final m was not a full consonant in classical Latin, but denoted nasalization and elongation of the preceding vowel. See this or this old question for more details. Was this effect limited to word-final position, or did an internal m nasalize the preceding vowel as well?
For example, how were sumus, summus, amplecti, sumpsisti pronounced? I would assume m had a fully consonantal nature between two vowels to avoid hiatus, but that does not rule out nasalization. There seems to be more freedom as the first consonant of a consonant cluster. For example, perhaps summus could be [sũ:mus] instead of the pronunciation [sum:us] I was taught. Sometimes the vowel before a consonant cluster beginning with m is already long (eg. sūmpsistī), which makes the combination of nasalization and elongation sound unlikely. The vowel before a final m is always short as far as I know — when m is pronounced as a consonant.