The sound loss is, in most cases, due to one of these two possibilities:
The Ring/Silent button on the side of the phone is pressed
The Ringer function is muted in Settings > Sounds & Haptics (or Sounds on older models)
If this solution does not solve the issue, please be sure to contact our Support by clicking on New Support Ticket so we can look into this problem! The Unknown Bug solution should help with this process!