When right clicking the menu and clicking volume mixer the mixer stretches across both of my screens, along with this when i move the window around the pc freezes and will not respond but the volume mixer window keeps moving.
-Info: Win 10 1903 / 9 playback devices / 1# screen = 1920x1080 / 2# screen = 1680x1050
if there is any other bits of info you need please say
The slow window dragging is #349.
This issue can track the window UX being awkward with lots of devices.
You can mitigate this issue by disabling devices if they are not necessary, but of course this doesn't help if you are using the devices (even passively).
can confirm OPs issue.
Having multiple monitors (btw GPU vendors removing DVI, instead adding HDMI doesn't help...)

Disabling isn't a permanent solution with driver and windows updates or the PC suddenly thinking "This is a brand new device! Let's enable it" re-enabling the devices every few months.
Would love to have either:
the volume mixer window resizeable (starting at a max width of my desktop)
or
the volume mixer window set to the max width of my desktop and make it scrollable
or
an option to show devices in a tiled style

If you look above your post I checked in a fix to add ScrollBars, it'll be in the next major update.