you have AXo, limp in middle position, NL HE

Flop comes AXB rainbow... A<X<B so you get middle pair with top kicker.
4 limp to see the flop, someone bets in front of you, and you have no read.

what do you do?

First, I agree with Marm. Playing Axo gets you into trouble more often than not. That said, I will admit that I am guilty of playing Axo from time to time.

Either raise or fold.
Fold: cut your losses, don't look back, find a better hand to play.
Raise: calling is pointless. You're just going to have the same crummy decision to make on the turn, only for more chips. (assuming your hand doesn't improve, which it likely won't)
If you raise, you are buying information. Its costing you the same as calling now and again on turn. 3 things can happen.
1) Maybe he folds. You win the pot and have a read on this player. He will bluff at the flop or lay down when he thinks he's beat.
2) If he comes back over the top, easy fold for you. And now you are set up for when you have a hand. Can't wait for later when he bets into your big hand, you reraise, and he comes over the top again.
3) He calls your reraise. Now the hand gets a little tricky, but you have position. If he bets into you on the turn, probably fold. If he checks, you can either bet to take the pot down, or check to see the river. A lot depends on the card that comes on the turn.

The problem is that if you hold A8 and the flop comes down

4 8 J

You may be up against AJ and so even making 2 pair is no good for you.
I would fold.