At my 1st appt (about 11 weeks) the Dr said we may or may not hear the heartbeat via doppler.
Most women do not hear/see a heartbeat at 6 weeks. It really depends on many different things: the skill of the sonogram technician, the angle of the uterus, where the baby implanted, the technology of the sonogram machine itself, etc.
A vast majority see/hear a heartbeat via sonogram for the first time around 6 1/2-8 weeks.
If you don't hear the heartbeat that early it does not automatically mean miscarriage. Just because you don't hear it doesn't mean that it isn't there.
If you're talking about hearing a heartbeat via a doppler at home, those are most accurate after 12 weeks.