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](https://latin.stackexchange.com/q/43/79) or [this](https://latin.stackexchange.com/q/1000/79) 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.