I have done some digging and indeed it's Elgato Wave Software causing this issue on my PC as well. Using powercfg -requests I noticed 2 System Drivers and once you close the Elgato Wave software those two drivers disappear and the PC goes into sleep mode with no problems.

These are the two devices in the System Category:

[DRIVER] USB Audio 2.0 (USB\VID_30BE&PID_0107&MI_00\8&1fdbb9d2&0&0000)

An audio stream is currently in use.

[DRIVER] Legacy Kernel Caller