When using wired headphones on iPhone alarm goes through phone speaker
when listening to a book or music using wired earphones, alarm goes through phone speaker
iPhone 12, iOS 17
Question marked as
This is normal.
Source: How to set and change alarms on your iPhone - Apple Support (IN)
"If you connect headphones to your iPhone, the alarm will play at a set volume through the built-in speakers on your iPhone as well as wired and wireless headphones."