var graph = new List<float>();
bool ok = mbApiInterface.NowPlaying_GetSoundGraph(graph.ToArray());
float[] graph = new float[500]
bool ok = mbApiInterface.NowPlaying_GetSoundGraph(graph);
or
float[] graph = new float[500]
float[] peak = new float[500]
bool ok = mbApiInterface.NowPlaying_GetSoundGraphEx(graph, peak);