I have little experience with SPA942 only and I have no one in production environment deployed anymore, but I has configured it the same way as recent SPA504G.
We has used 6.1.5 firmware only.
The like keys should show you which of it's line is being called:

But, may be, I missed your question.
What you mean "audio feedback to the handset earpiece" ? The volume of handset can be configured by user. Also, the volume of headset can be set.