In a theoretical, "anything's possible" sense of the word, then, yes, a machinegun could, conceivably pose a threat to even a Abrams. A very heavy MG (think light auto cannon, really) could manage a mobility kill; exposed optics and commo gear (just the antenna, really) might get damaged/destroyed, but those are mighty small targets.
By firing from "ambush," (if said ambusher managed to remain undetected; see below) any exposed crew might get hit, such as TC, Loader, or even Driver, if the driver's hatch is open.
But Abrams rarely work alone, and a firing machinegun stands out like a flare at night on the thermals, and the coax 7.62mm has a max effective range of 700 meters.
The TC's M2 has a max effective range of 1,800 meters.
And the main gun has a max effective range of 4,000 meters.
And one thing we learned in the first Gulf War is that those "max effective ranges" tended a tad towards the conservative side overall, but especially in open terrain.
It would take a very well crafted/concealed ambush, and more than a bit of luck, to get the drop on an Abrams or other similarly equipped MBT (Challenger, Leo II, AMX, Merk, T-90). I'm not saying that it can't happen; just saying it's damned difficult.
Our suicidal machinegunner has a lot of things working against him, however, and modern MBTs aren't nearly as myopic as some of you seem to think; even in '91 with just the Gunner's optical and thermal sights, we were "popping" Iraqi ambushes long before we entered effective ranges of their weapons systems.
The unmitigated can of whup-ass we sent at them left smoking, steaming, greasy smears on the ground. Night fighting was even easier; the desert was a tad chill at night even in the hottest parts of the year, and just the warm-air bloom from human body heat around even a well crafted/camouflaged ambush site gave away the enemy's positions.