GENKI Vocab Cards app doesn't play sound
Untoggle the mute switch at the top left of your iPhone. It will vibrate if it's muted. Unmuting won't make the device vibrate.

Make sure to increase the volume with the volume buttons.



Regarding Voice Memo.app

Tap the speaker icon at the top right to toggle the output (earpiece/loudspeaker).

Then adjust the volume with the volume buttons on the left of your iPhone.