WebAPI/AudioChannels

This API introduces the concept of a hierarchy of audio channels. The channels are prioritized as to allow "silencing all channels with priority lower than X".

The channels are: https://etherpad.mozilla.org/sound-stream-types

We'll have separate mute and volume settings per channel.

Open questions

Do we need the ability to play audio through the normal audio channel or through speaker even when headphones/headset is plugged in? That seems useful for alarms. Maybe the "alarm" channel never plays through headphones/headset?

API

interface AudioChannelManager : EventTarget {
 muteChannelsBelow(DOMString type);
 unmuteChannels();
 readonly attribute DOMString[] mutedChannels;
 attribute EventHandler onmutedchange;

 readonly attribute boolean headphones;
 attribute EventHandler onheadphoneschange; // Always fired before audio start playing through the new channel

 attribute boolean speaker;

 attribute DOMString volumeControlChannel; // The channel whose volume is changed if the user presses the volumeup/down buttons.
}