Glass's volume is set to 100% by default. To adjust the volume, do so here from the Settings bundle.
Volume set here and elsewhere determines the system volume, or the loudness of audio from your device's BCT (speaker). When earbuds are plugged-in, the volume settings will revert to the level set for your earbuds or the default level if plugged in for the first time. When you unplug earbuds, the volume will return to the previously set system volume level.