diff --git a/gumble/gumble/audio.go b/gumble/gumble/audio.go
index ceb10b2..5bde563 100644
--- a/gumble/gumble/audio.go
+++ b/gumble/gumble/audio.go
@@ -53,12 +53,10 @@ type AudioStreamEvent struct {
 type AudioBuffer []int16
 
 func (a AudioBuffer) writeAudio(client *Client, seq int64, final bool) error {
-	// Choose encoder based on whether buffer size indicates stereo or mono
+	// Always use the mono encoder for voice transmission. The stereo
+	// encoder is only used when explicitly enabled for file playback.
 	encoder := client.AudioEncoder
-	frameSize := client.Config.AudioFrameSize()
-	if len(a) == frameSize*AudioChannels && client.AudioEncoderStereo != nil {
-		encoder = client.AudioEncoderStereo
-	} else if client.IsStereoEncoderEnabled() && client.AudioEncoderStereo != nil {
+	if client.IsStereoEncoderEnabled() && client.AudioEncoderStereo != nil {
 		encoder = client.AudioEncoderStereo
 	}
 	if encoder == nil {
diff --git a/gumble/gumbleopenal/stream.go b/gumble/gumbleopenal/stream.go
index 819f35e..3fe561b 100644
--- a/gumble/gumbleopenal/stream.go
+++ b/gumble/gumbleopenal/stream.go
@@ -503,10 +503,21 @@ func (s *Stream) sourceRoutine(inputDevice *string) {
 					recorder.RecordAudioFrame(recorderOutgoingSource, outputBuffer)
 				}
 			} else if hasMicInput {
-				// Send mic when no file is playing
-				outgoing <- gumble.AudioBuffer(int16Buffer)
+				// Send mic when no file is playing. If the microphone is
+				// stereo, downmix to mono since Mumble voice transmission
+				// uses mono Opus encoding.
+				outBuf := int16Buffer
+				if s.sourceChannels == 2 {
+					monoBuf := make([]int16, frameSize)
+					for i := 0; i < frameSize; i++ {
+						// Average left and right channels
+						monoBuf[i] = int16((int32(int16Buffer[i*2]) + int32(int16Buffer[i*2+1])) / 2)
+					}
+					outBuf = monoBuf
+				}
+				outgoing <- gumble.AudioBuffer(outBuf)
 				if recorder := s.getRecorder(); recorder != nil {
-					recorder.RecordAudioFrame(recorderOutgoingSource, int16Buffer)
+					recorder.RecordAudioFrame(recorderOutgoingSource, outBuf)
 				}
 			}
 		}