I just testing in VirtualBox with Manjaro Xfce and there i see the tooltips on sliders fine.
So maybe you can tell what distribution and desktop environment you are using.
It seems your font size is very big in the top of the image so maybe its a font issue. Like maybe the font does not exists. But im just guessing.
I’m using Xubuntu 16.04 LTS (Xfce Desktop).
The font size is large because I have a fairly high dpi monitor, but Audacity isn’t dpi aware, so the font scales but the rest of the interface doesn’t.
Changing the font size and/or dpi make no difference to this problem.
A minor issue, but is this something that is being looked at for a future release? I have Audacity 2.2.2 running in a Singularity image (Docker for HPC) and I am also running into this issue. I personally rely heavily on tooltips while learning a tool so a fix would be great!