Hey,
I'm still using two DSM-750's after about 14 years! I'm using them as media extenders with Windows Media Center. I recently had some issues getting the DSM-750 to play audio content in 5.1 surround sound.
I finally managed to figure it out and posted a video on how to solve it on Youtube.
https://youtu.be/fOtYbBgCG-8
It turns out the problem wasn't with the DSM-750 but with codecs in Windows 7/ Windows media Center.
If anyone is still using a DSM-750 this video may prove helpful when solving audio issues.
Mike