I think it's a standard protocol at most hospitals here, including KKH where I did my scans. The sonographers explained to me that the machines aren't really set up for audio output during routine scans, and they keep the ultrasound exposure to the minimum needed just to check what they need to check. The flickering heartbeat is honestly enough to confirm baby is doing well, even though it would be nice to hear it. Some private clinics do offer Doppler heartbeat recordings as an add-on, but most of the restructured hospitals like NUH tend to stick to the visual confirmation only.