It doesn't seem to be an issue with speaker output because the meters don't even move.
Both previous projects (saved with b5) and new projects have the problem. The waveform on the audio track shows, but on playback the meters don't move and there's no sound. Media source viewer had same behavior.
Volume is full, not dimmed, and all tracks are at unity and unmuted.
Screenshot of my speaker settings:

- b6-speaker-setup.png (4.9 KiB) Viewed 6687 times
(This is correct: ADAT (3+4) is my system device.)
I changed speaker configuration to Manual, and configured the device accordingly. Suddenly audio works. I changed speaker config BACK to Use System Setting, and audio still works. It even survives a restart of Resolve.
If this is a specific peculiarity of b5 to b6 it's not much to worry about, but I suppose the bigger concern would be if this could be a user experience of upgrading from 14 to 15 final.