Avoid pops when pausing audio

[derat]

There's often an audible pop when pausing in the web client. I probably need to ramp the volume down (and also ramp it back up when playing?):

• https://stackoverflow.com/questions/21516490/audible-popping-when-pausing-some-html5-audio
• https://stackoverflow.com/questions/38387226/pop-noise-sounds-when-audio-paused-with-javascript

This doesn't seem to happen in the Android client. Maybe MediaPlayer already ramps the volume.