This is a short term solution to change the volume of an AudioTrack until applyConstraints for MediaStreamTracks has been implemented. This CL adds 1 new Java method & the relevant JNI file update: AudioTrack.java: public void setVolume(double volume); BUG=webrtc:6533 Review-Url: https://codereview.webrtc.org/2710683009 Cr-Commit-Position: refs/heads/master@{#17682}