Hi manuel.
You are right, at the moment the only way to play music via soundseeder is using your android phone.

A player version for java/raspberry pi would be nice, but (at the momemt) the android version is our main focus and porting it to “standard java” isn’t that easy as the porting of the speaker app was…

So I hope you can live with the current solution. 🙂
Another way would be, to get an Android TV box. You can get them for about 40€…