Hearing milk sloshing in a baby's stomach usually indicates that the baby has ingested too much air along with the milk. This is often referred to as aerophagia. The sound isn't the milk itself, but rather the air mixed with the milk causing a sloshing sound. This can be a sign of improper feeding techniques, such as an improper bottle feeding position or a fast feeding pace. Sources like the Cork Children's Clinic https://corkchildrensclinic.com/2022/04/25/paced-feeding-what-it-is-and-why-its-so-important-for-bottle-fed-reflux-babies/ and The Baby Reflux Lady https://www.thebabyrefluxlady.co.uk/blog/aerophagia-baby-reflux-colic highlight this connection between audible milk movement and excessive air intake. While some gurgling or sounds are normal in a baby's digestive system, https://www.bellybelly.com.au/baby/liquid-sloshing-around-babys-tummy/ a pronounced sloshing sound warrants attention.
Other sources mention that hearing swallowing sounds is a positive indicator of successful feeding, https://www.lllc.ca/sounds-breastfeeding while excessive noise during feeding could indicate air ingestion. https://milkmatters.org.uk/2011/04/15/hidden-cause-feeding-problems/
In short: Hearing milk sloshing is not normal and typically indicates the baby has ingested too much air during feeding.
