High frequencies??

Sarah

So I went to get an ultrasound yesterday, and initially although I could see the baby I couldn’t hear a heartbeat (they had the sound off) I asked if I could please hear the heartbeat as I have yet to hear it, and she initially said no? It was hard to understand her as she has a really thick like Russian accent but she kept saying something about frequencies? She finally let me hear it after I became visibly upset, but I wasn’t able to hear it for long nor was I told anything about it other than everything looks good. Has anyone else ever been told this? I’m just wondering what it means.