I'm afraid so... for now. The track "level" (Index or DisplayIndex)
cannot be set after it is created. However, if you are creating the
track from your script you can specify the index in the Track object
constructor.
But the code for creating and adjusting the composite level envelope
is:
var compositeEnvelope = new Envelope(EnvelopeType.Composite)
track.Envelopes.Add(compositeEnvelope);
compositeEnvelope.Points[0].Y = levelValue;
where the track variable is your target Track object and levelValue is
an appropriate double value that is between the envelope's Min and Max
properties.