My apologies for the delayed response,

I can confirm that reloading a whitelisted tab toggles the mute state, which seems to be related with the "calendar.google.com" issue and I'm currently working on it.

About the "about:newtab" for me "about:blank" did the trick.

About the regex you need to use slashes (/) at the start and beginning of the rule for the extension to recognize it as regex rule.