You know, I swear I could hear a heartbeat one time with my pregnant mare. I just had my ear to her gut, when the foal was pushing on that side (I could see the bulge moving so I knew he was near that spot in her belly). And I wasn't using a stethoscope even. It sounded like a heart beat only faster than my own (and I assume the mare's wouldn't be that fast either). So I too would like to know, are you supposed to be able to hear the foal's heartbeat? It sure sounded like it to me, but I have only heard it once, and since I don't have a stethoscope, the odds aren't in my favor of hearing it again!
You might also want to try watching her belly (if you have the time) because my mare's baby moved around a lot. He actually moves less now that she is closer to being due (maybe because there's less room in there?) but I'm thinking around the 9th month or so, he started kicking around a bunch!