There are reports that built-in speakers of iPhone 15 produce crackling sounds while playing audio in some cases. At this moment it is hard to evaluate how widespread the issue is. Issue seem to be affecting both Pro and non-Pro models.
According to reports, high volume playback of high frequency sounds could be triggering the issue.
At this moment, Apple seem to be replacing iPhones affected by this issue at no cost.
Reference: Some iPhone 15 users complain about crackling sound with built-in speakers