My clip volume keyframes have defaulted to a bezier, I cannot make them linear.

Thanks all for replies, good to know its just a dsiplay bug. I was going crazy traying to figure out what was happening. 

This is only a display error. This is actually a linear fade. Look here:
https://community.adobe.com/t5/premiere-pro/using-pen-tool-to-adjust-audio-track-volume-in-premiere-is-creating-a-curve-automatically/m-p/11620331
https://community.adobe.com/t5/premiere-pro/audio-clip-keyframe-won-t-fade-linear-even-though-it-s-selected/m-p/11621847

Delete the Media Cache and Media Cache files:
https://community.adobe.com/t5/premiere-pro/faq-how-to-clear-your-media-cache-in-30-seconds-in-premiere-pro-2020/m-p/11017257/thread-id/260446?page=1

If that doesn't work,try resetting the preferences:
https://community.adobe.com/t5/premiere-pro/faq-how-to-reset-trash-preferences-in-premiere-pro/td-p/8236158?page=1

If that doesn't work, try resetting the Workspaces:
Reset a workspace
Reset the current workspace to return to its original, saved layout of panels.
1. Do one of the following:
• Click the Workspace menu icon and select Reset to Saved Layout.
• Choose Window > Workspace > Reset to Saved Layout.
from here:
https://helpx.adobe.com/premiere-pro/using/workspaces.html

If that doesn't work, try Preferences > Audio Hardware and set Input to None.

If that doesn't work, try creating a new project and import the old one into it.

Also, please help us help you:
https://community.adobe.com/t5/premiere-pro/faq-what-information-should-i-provide-when-asking-a-question-on-this-forum/td-p/3929027?page=1