This app was mentioned in 7 comments, with an average of 2.14 upvotes
FIGURED IT OUT!
It was so simple im facepalming.
Download a free or paid music visualizer:
https://play.google.com/store/apps/details?id=com.h6ah4i.android.music_visualizer2&hl=en
Or if you want a more Milkdrop like experience purchase this one:
https://play.google.com/store/apps/details?id=com.psperl.projectM&hl=en
Play music on phone > open chromecast app > cast android devices screen. Sound and visualization is then on the TV.
That one is great, I also use Music Visualizer.
https://play.google.com/store/apps/details?id=com.h6ah4i.android.music_visualizer2&hl=en
I would pay for 'pro' or an 'unlocker' just for this feature.
If the visuals in these apps:
https://play.google.com/store/apps/details?id=com.h6ah4i.android.music_visualizer2&hl=en
https://play.google.com/store/apps/details?id=com.cyphercove.audioglowfs&hl=en
were viewable in cardboard, i would be ecstatic
If you have android:
https://play.google.com/store/apps/details?id=com.h6ah4i.android.music_visualizer2
I use this. It works with external player apps and the visualisations are customisable.