Media server daemons, running with a high privilege in the background, are attractive attack vectors that exist across various systems including smartphones. Fuzzing is a popularly used methodology to find software vulnerabilities although symbolic execution and advanced techniques are obviously promising. Unfortunately, fuzzing itself is not effective in such format-strict environments as media services. Thus, we study file format-aware fuzzing as a technical blend for finding new vulnerabilities. We present our black-box mutational fuzzing on the latest smartphone systems, Android and iOS, respectively, with manipulation of the MPEG-4 Part 14 file format and show results that affect a wide range of related systems. In our approach, we automate a seed file selection process to crawl a crowd-sourcing public website and validate arbitrary m4a/mp4 audio files according to the FOURCC atom list we gained through white-box analysis in Android. We acquired eight seed files covering all effective atoms in 2,600 s. We then performed size field mutation in a little amount and generated 1,102 test cases common to both systems. During six CPU hours of fuzzing, we identified three crash atoms in iOS 9.3.5 and 15 in Android 6.0.1, respectively. Due to format-awareness, we were able to easily locate crash points through a mutation table. It was discovered that the new crash atoms found in iOS allowed remote attackers to execute arbitrary code or cause a denial of service by memory corruption in iOS and also OS X, tvOS and watchOS.