Slacker played absolutely nothing when I requested a song. It parsed my speech and returned no results on the search. Tesla provides a Slacker Plus subscription with the car. Once I logged in with an account that still had an active Plus subscription, everything started working properly again.Slacker will not always play the song you request. It has limits on how many times a song can be played within a specific time limit. If the counter is exceeded, it will pick a song that it thinks similar.
If you want to be guaranteed to play a specific song (if in the catalog), you would need slacker plus.
As long as the car is displaying the correct song on the dash, it is probably decoding your speech correctly.
You are correct that there's a cool down on requesting the same song. I haven't tried upgrading to a Premium account to see if that cool down is removed or not.
I can only assume that the Slacker Plus subscription expired on the account my car was logged into. Without knowing the password, there's no way to know for sure and it wasn't the account that Tesla seemed to think I should be using.