Q: What happens if I'm playing a RF64 file in Windows Media Player?
A: RF64 files can be larger than 4 GB. Those files are not currently supported by Windows Media Player and similar products.

Smaller files can be read be Windows Media Player but the playout will be limited to 5.1. The downmix will be done by functions of internal Windows Media Player which means that the existing stereo track won't be used for playout.
If you've got a 5.1 audio card in your PC you will get a 5.1 playout.