
Palm muting is indicated with "P.M." in tablatures. What's interesting is that you will see this abbreviation in 25% of Songsterr's tabs. To hear how palm muting sounds on Songsterr, you can check Basket Case by Green Day.

And another small addition - "let ring" effect indication. While all songs on Songsterr actually supported "let ring" effect for a very long time, you could only hear it - as there was no visual indication.
From now on, if you see the "let ring" label in tablature you will know that it's really there. You can check Wanted Dead Or Alive by Bon Jovi as it uses "let ring" heavily in the intro.