Forgive me but I miss “solo” and “unmute all”… Maybe I am too stupid to find those buttons…
They are only in the Mixer on on top next to the AutoSolo control.
There are so damn more ways to mute/solo tracks in Renoise (right click in the mute icon in the pattern for example, the track scopes), so I really wonder that you miss it…
Okay, I was used to do it this way so I was wondering where those buttons went to. There’s still enough space for them, IMHO.
But it’s okay for me, if it stays it this way…
Rightclick on the scopes should do this trick as well.
Of course it does. But this has been there for ages and so I stumbled over it. And as there’s still enough space, I was wondering “why” 
Perhaps try set up some keys as well 
AFAIK “unmute all tracks” is unassigned by default.