Just a Workaround until Apple posts a fix…
Using Rules in Mail I just wrote Apple Scripts to perform the functions I needed:
Here is an example of how to play a sound for New Mail:
Using the "Script Editor.app" in Applications/Utilities
Just open then select "New Document"
Enter this script:
tell application "Mail"
do shell script "afplay '/Users/MyAUDIO/BeepBeepBeep.wav'"
end tell
You will NEED to change the Path and the name of the sound file to one of your liking.
Use the "Play" arrow to test if the sound plays and test for script errors.
Just save the file to: "~/Library/Application Scripts/com.apple.mail" folder with a name like: PlaySound.scpt
Now, just go to the Mail "Rules" and use "Run AppleScript" for your Action.
If you placed your Script in the Folder above then it will be listed in AppleScripts Dropdown when creating the Rule.
Use something simple like: Check for "Any recipient" "contains" "@"
You should be able to get all your Rules working by creating an Apple Script to Perform the Action Instead using the "Built-in" actions that no longer seem to work...
To prevent Multiple Sounds from triggering at once:
Just add the Action "Stop evaluating rules" after the AppleScript Action.
Also, Note that “Rules” process from Top to Bottom so if you want multiple rules to process then Drag them to the Top and move Sound Alerts to the bottom with a "Stop evaluating rules" as the Last Action within the Rule.