The project you’ve attached plays back just fine, so that suggests that the sound content and the HALion Sonic SE plug-in are all installed correctly. You should find that installing the FlexASIO driver has no impact on any other applications on your computer, but I can understand your reluctance to interfere with an otherwise working system.
Have you been through the playback troubleshooting video?
(There’s a transcription of the text of that video a couple of posts further down in that thread.)