Hmm so I am not the only person with crackle sound issue too. I have my iPhone 8 Plus since launch date, noticing this crackle only from speakers in a few apps e.g. 4 Pics 1 Word puzzle game. Don't notice it when playing music. Brought my phone to an Apple store last week and shown to an Apple genius, able to demonstrate the crackle sounds from that game to her, but she had no idea what could be causing it. Suggesting may be the app is the source of problem and needed update from the developer. I had an iPhone 6 before this and it never had this crackle sound issue.