It’s a little-known feature of most Macs that the 3.5 mm headphone port actually doubles as a optical S/PDIF port, which means that given the right cable, you can get a high-quality PCM audio stream right into the optical input on your stereo receiver. Unfortunately, since macOS lacks native support for advanced codecs like Dolby Digital AC-3, you’ll be limited to two-channel audio unless you’re using an app like DVD Player. This is where Coriolis Systems’ new Aura comes in — it’s a little $20 utility from the developer known for iDefrag and iPartition, which runs in the background to encode multi-channel audio from your Mac into a Dolby Digital AC-3 stream that can then be handled by any compatible receiver.
While there aren’t a ton of apps that support 5.1 channel sound on the Mac, it’s still a cool idea, and you can easily check it out with some Test Tones from Dolby Labs. Nevertheless, native 5.1 Dolby Digital AC-3 support is something we’ve been waiting for on the Mac for a long time, and it actually turns out that the real reason has been the vagaries of patent and licensing issues — Aura was actually ready four years ago, but it’s only recently that it’s been possible to actually release it.