Good, now the problem has been isolated to optical. Getting a higher card will not likely change volume or surround effect. Some quality may improve but the problem lies in configuration.
Soundbars can give surround effect only in small narrow rectangular rooms. Do you have such a room? But still can't expect surround effects like true 5.1.
Next, as dinesh suggested, have you tried configuring audio on pc? In Windows 10 it is at the right end of the taskbar. Right click and check the options. Post the screenshots here.
Now, coming to HDMI. The HDMI input in magnifi mini is supposed to be connected to hdmi arc input of TV and extract audio from it. It is not built to be connected to hdmi out of pc, bluray player, console, etc. But you can try it. When you connect the soundbar via hdmi and monitor via display port, are both working? If monitor only working and no audio, then again try configuring audio on pc. Post screenshots.