Foetal heart rate slows generally towards the end of gestation, a 37 week baby might have a heart rate of 150 bpm but a 42 week baby is likely to have one closer to 120 ( roughly) in addition, like your heart rate, the foetal heart rate will vary according to foetal activity levels, position in relation to the cord/pressure on head/ contractions/ maternal position/ maternal temperature/ maternal heart rate......
It’s impossible to pick out whether the baby is a boy or a girl from a heart rate alone.
The different sounds and rhythms are also misleading, a heart can sound different depending on the position of the ultrasound device and the area it’s pointing at - the device then converts the ultrasound signal to an audible signal. Different monitoring methods create varying sounds, using a pinard stethoscope a foetal heart sounds like a very far away drum beat, whilst a foetal scalp electrode will convert the signal to a beep.
It’s all old wives tales - sorry