The mplayer package in [extra] do not make use of libfaad2, an AAC sound decoding library. The support for this wasn’t compiled in when the binary package was built. When mplayer builds, it checks what’s available on the system and make use of it. This means libfaad2 wasn’t on the system mplayer was built on. Therefore, the faad2 package should be added as a dependency of the mplayer package.

I believe this happened because in the past, mplayer used a forked internal copy of faad2, so we didn’t need to have it as a dependency [1]. As an alternative, mplayer can also use the external libfaad2. But recently, they dropped their internal fork because they made ffaac the default. Ffaac is the ffmpeg AAC decoding library. However, I’m unable to play the sound in some videos with ffaac, whereas faad2 runs fine. So I believe it’s worth keeping support for libfaad2.

[1] See https://bugs.archlinux.org/task/4065.