The volume cannot be adjusted in the sauna, but only via the connected end device. If the lowest volume is still too loud, please check the internal volume settings of the device and also the settings of the music player you are using (e.g. Spotify or Youtube).
If none of these lead to the desired result, we recommend the app “Volume Control Pro” for iPhones or “Precise Volume” for Android phones, which can be used to finely regulate the volume.