Check the volume settings. The connected device may have been muted, resulting in you being unable to hear any sound from your Ear (stick). If so, adjust the volume to an appropriate level.
Check whether this issue persists with other apps. If not, adjust the sound settings under the specific app settings.
Disconnect your Ear (stick) from your device. Place your Ear (stick) back into the charging case, close the charging case and open it after 5 seconds, and then connect your Ear (stick) to device again.
If the issue cannot be resolved, take your device and proof of purchase to an authorised Nothing Service Centre for assistance.