Verizon's latest update for the Moto Z and Moto Z Force makes the ringtone louder and adds the latest Android security patches.
The update, as denoted on Verizon's support page, fixes the default notifications volume level so that it matches the level that was used in the original software release. It's unclear whether this fix is directly related to the known low volume issue. However, users in our forums have reported success after the software update.
The update is software version NCL25.86-11-4-6. The update also includes the latest Android security patches. You can download it by checking for a software update on your Moto Z or Moto Z Force.