Considering the speakers you use, that may be the issue. You see, the Z5500 has a very only decoder, I just looked and it advertises the following:
"The possibilities for decoding as such include not only the classic Dolby Digital, but also DTS and the less common 96/24 (24 bits/96 kHz). Add to that Dolby Pro Logic II, which lets you decode an analog source with MP encoding (called Dolby Surround or Dolby Stereo) and also go from a stereo source to 5.1 with cinema or music modes and the usual adjustments. Unfortunately the corresponding mode for DTS (Neo:6) is absent."
No mention of Dolby Digital Live/Dolby Digital Plus or any of the recently used Dolby encodings. Basically, that sound card may be sending out a 5.1 encoded signal using a different algorithm that your speakers are just too old to support.
Asus G20CB - Core i7 6700, Nvidia GTX980, 16GB Ram, 256GB SK Hynix SSD, 1TB Samsung SSD.