This deal actually has been going on since late-November but is probably ending soon (said they'd be going to the end-of-the-year 2015).
http://dysonics.com/give-the-future-of-audio-this-holiday-season/
Dysonics RAPPR is $4.99 typically it is priced at $14.99. Essentially, this will decode your 5.1 and 7.1 surround files, and it does quite a good job of recreating the surround sound for your 2-channel stereo headphones.
I snapped it up today, having used a trial in the past. The only major problem I had then, was I was getting distortion using it through any USB DAC. I sent them a ticket then, so hoping this is cleared up. I don't mind using my internal Macbook out, but this should be a simple fix.
They also have hardware called RondoMotion that will change the soundfield relative to your head position. It works in conjucntion with the software.
Update: well, disappointed to say that perhaps this isn't the best purchase. Even at $4.99. There is still the issue using an external soundcard where the bass is distorted. It's almost as if they aren't compensating for the above line-level processing. Basically,its clipping and digital distortion which is not good. Using the internal soundcard is fine on Mac, but this is a no go using a separate DAC.
Also, the sound quality is not that clear as a product like Hajo's Headphone Enhancer.However, as of the latest version of Hajo's v1.7.9 there is still some digital distortion going to an external DAC, it can be mitigated somewhat depending on the setting or using a custom setting and adjusting input/output to avoid clipping.
