Using GetVolume() returns your device volume value, but the scale may vary due to your device model. Scaling this value in the range of (0 to 1) may be like this:
B4X:
Public Sub GetVolumeScaled() As Float '0 to 1
Dim myPhone As Phone
Return myPhone.GetVolume(myPhone.VOLUME_MUSIC) / myPhone.GetMaxVolume(myPhone.VOLUME_MUSIC)
End Sub