exp/audio: fix inability to use mono 8 format

There is no way to distinguish an unknown format from
Mono8 because Mono8 is zero. The current CL fixes this
issue by starting the format constants from iota + 1.

Change-Id: I8c709cdaff3bc46bcc15ab770cfcfd04c186034d
Reviewed-on: https://go-review.googlesource.com/12386
Reviewed-by: Burcu Dogan <jbd@google.com>

exp/audio/audio.go

@@ -41,7 +41,7 @@ type ReadSeekCloser interface {
 type Format int
 
 const (
-	Mono8 Format = iota
+	Mono8 Format = iota + 1
 	Mono16
 	Stereo8
 	Stereo16
@@ -65,6 +65,7 @@ var formatCodes = [...]uint32{
 }
 
 var formatStrings = [...]string{
+	0:        "unknown",
 	Mono8:    "mono8",
 	Mono16:   "mono16",
 	Stereo8:  "stereo8",