Nobody can tell you anything without proper system info or other technical details like your system audio configuration, AE's audio prefs and so on.
If you use TR, then it's probably just yet another bug with the realtime preview stuff. Pre-compose your clips to enforce caching to RAM.
Hmm, no luck, is this just a bug with the new previews?
I am very looking forward regarding this case.
I have an Macbook I7 8Gb RAM El captain installed and running the trial version of AE CC 2015.
If it does`t work I will not buy it no more.
*** I used to use the AE CS6, but I˜ve had problems with ˜Not running real time˜...
Please, if anyone get some tip regarding it, I really appreciate that!
Bruno Moraes @Brazil
i saw you had mp4 in your timeline.
perhaps first transcoding to prores, dnxhd, cineform with prelude will remove the offending error box.