Using NAudio, you could do the following:
using NAudio.CoreAudioApi;
.....
// download NAudio.dll from https://github.com/naudio/NAudio/releases
// and add it as Reference to the project
var devEnum = new MMDeviceEnumerator();
var defaultDevice = devEnum.GetDefaultAudioEndpoint(DataFlow.Render, Role.Multimedia);
var volume = defaultDevice.AudioEndpointVolume;
float leftVolumePercent = volume.Channels[0].VolumeLevelScalar * 100;
float rightVolumePercent = volume.Channels[1].VolumeLevelScalar * 100;
float masterVolumePercent = volume.MasterVolumeLevelScalar * 100;
This article demonstrates, how to achieve such things without external frameworks.
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solved How to get Speaker Volume Level? [duplicate]