I may be posting in the wrong place...so sorry in advance!

So I recently bought a Chromecast principally to stream Netflix from laptop to big TV without having to use cables. (Router was fiddled with to faciliate this!)
The big TV I have is a Panasonic Plasma 42" (2010 vintage) and I also have a Panasonic Blu-Ray Home Theatre system (SC-BTT300 - entry level but about 2-3 years old). I channel all my audio via the Home Theatre so don't use the speakers on the TV at all.
The issue I'm having is that whenever try to cast something from the laptop to the TV via Chromecast, the Home Theatre system goes nuts and acts like it is looking for a DVD or Blu-Ray disc to try and play. It does this for a few seconds, goes to the home screen, then goes looking for a disc again...and does this on a loop. I've tried this with Netflix, the Plex app, and YouTube and exact same thing happens. The only way I can get sound is to use the TV speakers and switch off the Home Theatre system.
The really weird thing is that if I use my iPhone and cast Netflix or Youtube from there, everything works perfectly and the sound comes out of the Home Theatre system with no issue. This makes me think there is some sort of setting in the laptop that I need to change but I cannot find it.
Any pointers would be hugely appreciated as I want to get full value for it!!