I didn’t hear my babies heartbeat until about 18 weeks, only saw it on the screen during the scan
At my scan today (9 weeks) the sonographer said they don't turn sound on that early and just showed us the heartbeat on screen. Maybe that's why your doctor wasn't concerned - it might be quite unreliable early days.
It was the same with me...I just saw the heartbeating,but I couldnt hear it...I was with 6w
Heartbeats in very young fetuses are extremely hard to find by Doppler. This is why some trusts no longer offer to use Dopplers during midwife appointments from 16 weeks