The website tomahawk-player.org maybe has an expired certificate. Should I ignore that?
According to sources that project has been "essentially abandoned" (nobody is working on it)

So, even if you download it, you are on your own with any problems that might occur.

As for accessing websites with expired certificate, you have been warned not to proceed, but you can at your own risk, witch could be major or minimal.

If you wish to proceed you can download the player from here 

Just tried that and it Opens fine on HighSierra