Index: app.c
===================================================================
--- app.c	(revision 18302)
+++ app.c	(working copy)
@@ -555,6 +555,7 @@
 	int gotsilence = 0;		/* did we timeout for silence? */
 	int rfmt=0;
 	struct ast_silence_generator *silgen = NULL;
+	int audio_avail = 0;
 
 	if (silencethreshold < 0)
 		silencethreshold = global_silence_threshold;
@@ -652,6 +653,7 @@
 				f = NULL;
 				break;
 			}
+			audio_avail = 1;
 			f = ast_read(chan);
 			if (!f)
 				break;
@@ -735,7 +737,7 @@
 	if (silgen)
 		ast_channel_stop_silence_generator(chan, silgen);
 
-	*duration = end - start;
+	*duration = audio_avail ? end - start : 0;
 
 	for (x=0;x<fmtcnt;x++) {
 		if (!others[x])