TrackBar volume control
There isn't much to review in 6 lines of code. But oh well.


Native methods return error codes, so it is good practice to handle those codes somehow, not just ignore them. For example, if method reports, that there is no sound driver or no sound card or w/e, it makes sense to notify user somehow. Write warning in the status bar of your window or something.
It is meaningless to initialize CurrVol, since its value will be overriden anyway, wont it? And, if its your real code, you dont have to recieve current volume in order to set it to begin with.
Convert.ToString should be refactored to String.Format("Volume: {0}%", trackBar1.Value);. More complex example:

var status = String.Format("Current Buffer: {0} Clear: {1} Ping: {2} Buffer: {3}", waveProvider.BufferedDuration.Milliseconds, TimesBufferClear, PingReply, SendStream.BufferMilliseconds);