In Australian English, calm rhymes with farm and uses a long vowel, while com uses a short vowel and would rhyme with prom. (I know this doesn't help much because some American accents also rhyme prom with farm).
It's not 'calm' that differs, it's 'common'. Calm like palm, in all major accents.
Traditionally, calm and com- have different vowels in English, but most North American accents merge com- into calm. All other major English accents retain the distinction.
If you're American, try saying 'com' while rounding your lips. Or just listen to a recording of 'common' in an online dictionary from Britain or Australia. (Or lot, pot, spot, etc.)
I've been thinking about this for a minute, and I think if an American were to say "why", and take only the most open vowel sound from that word and put it between "k" and "m", you get a pretty decent Australian pronunciation. I am an Australian so I could be entirely wrong about how one pronounces "why".