Fix mixed audio messed up because of sleep in mutex lock

src/main.cpp

@@ -3287,6 +3287,7 @@ int main(int argc, char **argv) {
         amix_thread = std::thread([&]() {
             AVFrame *aframe = av_frame_alloc();
             while(running) {
+                {
                     std::lock_guard<std::mutex> lock(audio_filter_mutex);
                     for(AudioTrack &audio_track : audio_tracks) {
                         if(!audio_track.sink)
@@ -3306,6 +3307,7 @@ int main(int argc, char **argv) {
                             audio_track.pts += audio_track.codec_context->frame_size;
                         }
                     }
+                }
                 av_usleep(5 * 1000); // 5 milliseconds
             }
             av_frame_free(&aframe);