Change opus min bitrate.

webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc

Index: webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
diff --git a/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc b/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
index bd0afc6fa8eeda15a65592bacf9b29e22faf9742..6b39cc8ac74f7b061877ae607557c4d4d4521ea3 100644
--- a/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
+++ b/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
@@ -29,7 +29,11 @@ namespace webrtc {
 namespace {
 
 constexpr int kSampleRateHz = 48000;
-constexpr int kMinBitrateBps = 500;
+
+// Opus API allows a min bitrate of 500bps, but Opus documentation suggests
+// a minimum bitrate of 6kbps.
+constexpr int kMinBitrateBps = 6000;
+
 constexpr int kMaxBitrateBps = 512000;
 constexpr int kSupportedFrameLengths[] = {20, 60};