Sometimes when I park the cursor on a spot in a timeline (usually to check the audio at a chapter point) the audio does not play back. Actually, I get a strange hum. Then, I park the cursor elsewhere and the audio plays back normally. Something I'm doing, or a bug?
To work around it, don't transcode audio until you're ready to build the project (File > Revert To Original if it's already transcoded)- or simply reset the CTI in the timeline and try again, as you found.