What To Do
Assess your horse's general health using the Whole Horse Exam (WHE), and look for other signs of illness, particularly those associated with abdominal pain (colic). Listen to all quadrants again in quiet surroundings, using good technique and a good stethoscope. You may simply not have been able to hear the sounds.
If your horse has decreased intestinal motility, but shows no other signs or illness or disease, they may be just fine. Offer them some feed. If you then reassess their gut sounds and notice increases in intestinal sounds, you may simply monitor them.
Contact your vet if your horse is showing other signs of illness or disease. In many cases of abdominal pain, regardless of cause, there is an early period of reduced gut sounds in at least one quadrant of the abdomen and monitoring the signs of intestinal motility is a useful indicator.