Я знаю, что этот вопрос действительно старый, но у меня это сработало:
http://www.dotnetcurry.com/showarticle.aspx?ID=431
Imports System
Imports System.Windows.Forms
Imports System.Runtime.InteropServices
Namespace WindowsFormsApplication1
Partial Public Class Form1
Inherits Form
Private Const APPCOMMAND_VOLUME_MUTE As Integer = &H80000
Private Const APPCOMMAND_VOLUME_UP As Integer = &HA0000
Private Const APPCOMMAND_VOLUME_DOWN As Integer = &H90000
Private Const WM_APPCOMMAND As Integer = &H319
<DllImport("user32.dll")> _
Public Shared Function SendMessageW(ByVal hWnd As IntPtr, _
ByVal Msg As Integer, ByVal wParam As IntPtr, _
ByVal lParam As IntPtr) As IntPtr
End Function
Private Sub btnMute_Click(ByVal sender As Object, ByVal e As EventArgs)
SendMessageW(Me.Handle, WM_APPCOMMAND, _
Me.Handle, New IntPtr(APPCOMMAND_VOLUME_MUTE))
End Sub
Private Sub btnDecVol_Click(ByVal sender As Object, ByVal e As EventArgs)
SendMessageW(Me.Handle, WM_APPCOMMAND, _
Me.Handle, New IntPtr(APPCOMMAND_VOLUME_DOWN))
End Sub
Private Sub btnIncVol_Click(ByVal sender As Object, ByVal e As EventArgs)
SendMessageW(Me.Handle, WM_APPCOMMAND, _
Me.Handle, New IntPtr(APPCOMMAND_VOLUME_UP))
End Sub
End Class
End Namespace