Add this function in a subclass to get a notification just before an audio clip is being imported.

	class MyAudioPostprocessor extends AssetPostprocessor {
		function OnPreprocessAudio () {
			// Import files containin "mono" in their name as mono.
			if (assetPath.Contains("mono")) {
				var audioImporter : AudioImporter = assetImporter;
				audioImporter.forceToMono = true;
			}
		}
	}