So this surprised me. If I use the volume switch with nothing playing to turn "sound effects" volume up really high, then call AudioServicesPlaySystemSound(), I get a nice loud sound…
…unless I'm playing audio in some other app like Music or Pandora. In that case, the system volume attenuates the system sound volume. So, even if the sound effects volume had been set really high, if I'm playing some Apple Music or something and have its volume turned down, then the sound effect is much quieter than it would be if the system sound were playing by itself.
Is there any way to preserve the sound effects volume behavior, regardless of whether other audio is playing? Doesn't seem like the session category makes a difference.
—Chris