Well Hobie, to quote a local beekeeper here in Maine, don't bother checking your hives in mid-winter, if they're dead, they'll be just as dead come spring.
Of course, last winter- my first- I checked my hives almost weekly. This year, not so often. I was last up looking at them in late December when they were flying around. I haven't been up since. The next day it gets up to 35 or so I'll probably go look at them.
If you can look in the entrance and see some dead bees on the bottom, that's OK. If the hive was dead you'd probably see LOTS of dead bees on the bottom. 20 isn't anything to worry about. House keeping falls down during the winter.
As for hearing the bees.. my hearing is lousy I guess. I usually can't hear the bees during the winter without using a stethoscope and at that, it's a pretty faint hum. Try listening to an empty box and comparing that with what you're hearing. I wouldn't crack the lid if I could help it- they've no doubt got it glued down, for a good reason. Do they still have stores? Is the hive heavy?