Can your conversation partner not hear you?
Check whether you have authorised the app to use your microphone. You can find this in the settings of your device under the section Luscii. Make sure that the slider next to Microphone is green.
Can you not hear your conversation partner?
Use the volume keys on your device to turn up the volume and try using earplugs. Also ask your conversation partner to check whether he/she has authorised the Luscii app for the microphone.
Still don't have sound?
Log out of the app, then close the app and delete the app. Then reinstall the app and try video calling again.
Good to know: Did you install Luscii on your smartphone and were you making a call when you suddenly got a call? You can't just make calls and image calls at the same time on the same smartphone. Hang up both calls, count to ten and try image calling again.