"First take a selfie, then use the Meitu app."
The 用美图, 自拍, 美图 and 先自 Word Bank tiles are silent, yes, four out of nine. Such tiles always stay silent for the entire lesson. Defective network code? Unusable, time to drop duolingo. Reported [The audio does not sound correct] 2019 June 23rd Sunday Opera 60.0.3255.170 Mac OS X 10.11.6 64-bit.
UPDATE: Having quit duolingo, in November I checked in and to my surprise, not only were the audio bugs fixed but the [Report] menu added checkboxes for SILENT and OTHER, thanks duo team! Well, after a few weeks of lessons I suddenly encountered silent tiles every time (e.g., [先自] here) so I'll quit again and check back next year.