About 90% of the time, audio doesn't show up AT ALL here in a RAM preview. I'm always surprised (and delighted) when it DOES.
I'm having to render out successive movies to hear how well (or not so well) the audio actually works. When I do hear those little stops and starts when RAM preview is being built, I am excited, because I know I will hear the audio when it's finished. :+)
It's all buggy, at this stage. Probably will be until mid next year.
Yes, I do mean mid 2017.
If you update to the latest version, there is an audio button in the preview panel where you can turn off the audio during RAM previews. If you want real-time playback, try mousing over the playback window and hitting the ~ tilde key to solo the panel. By hiding the timeline and the moving playhead it might work. I have given up on even hoping for real time playback or audio and would love for CC2015 to not crash on me or stop grabbing layers and key frames and not letting go. I have a brand new iMac and it shipped with El Capitan. CC2015 is unusable in a professional studio and AE CC 2014 is more stable. I have found CC 2015 to be more reliable on a cheese grater Mac Pro tower running Mavericks. Our hope is to have a working 2015 in 2016!
Adobe After Effects CC 2015.1 version 22.214.171.124
OS X El Capitan
iMac (Retina 5K, 27-inch, Late 2015)
Processor 4 GHz Intel Core i7
Memory 32 GB
Graphics AMD Radeon R9 M395X 4096 MB