I haven't used SharpDX so far, but looking at the online documentation about BufferStart event definition, you can try to attach event handler this way :
sourceVoice.BufferStart += sourceVoice_BufferStart;
private void sourceVoice_BufferStart(IntPtr e)
{
....event handler actions...
}
Can't help with complete answer, just a slight push that could probably fix the error. posting links to other stackoverflow posts you referenced could also be helpful.